As someone who has played MS since the open beta ended way back when up til a few months ago, and even played the Korean, Japanese and SEA servers at different points in time, I think I can give a prety good overview.

A lot of people are blinded by nostalgia when it comes to this game. You may even see people saying that the game was better back when it took 12 hours to lvl up at lvl 73, which is absolutely absurd. The grind to even play your character back then was absolutely ridiculous. The only thing that pre-Big Bang Maplestory had that this version now doesn't is the sense of community and life that was everywhere. People will say that there isn't a sense of community now and that people don't help each other which is untrue. The community just isn't at the low levels, low levels meaning anything below 200. The players are still very friendly and helpful, and will go out of their way to party you if you have quests on the same map, and the Chu~Chu Island PQ at lvl 210 is a bunch of fun. Except for birdsharks. ♥♥♥♥ birdsharks.
You've seen me say lvl 200 and 210 previously, so how many levels are there? 250. There are 250 lvls in this game and at no time does the progression ever not give you rewards. 1-140 give you skill points, 141-200 give you hyper skills, 141-250 give you hyper ability points, and every 6 levels after 200 gives you an extra node slot. The first 200 lvls fly by, there are people that can do them in 8 hours and for some it'll take a few weeks. Most notably though is the insane Exp curve. At lvl 200 the Exp required to level goes from about an hour or two to 4 hours or more, especially if you play on Reboot and train in the Arcane maps before getting the 1,5x Arcane force multiplier from the daily quests. And the grind only gets more and more difficult. Previously I mentioned that at lvl 73 pre-Big Bang you could take 10+ hours to level up. Post Big Bang getting from 203-204 takes about as long without the Arcane force multiplier affecting your damage and letting you grind at better mobs. Because as an unfunded character you're going to be grinding at either Scrapyard or the first area of Vanishing Journey for a good 2 or so weeks if you don't miss any Vanishing Journey daily quests. Later on you can expect 2% exp per hour, which, considering the lvl requirements of later areas like Arcana, is going to take you months.
As for the social aspect of the game now, joining a good, active guild is necessary. If you don't join a good, active guild, on Reboot especially, your progression past 200 will be absolutely glacial. You need a good guild to do end game bosses with and to get decent equipment, which is doubly true for Commerci PQ because most people aren't going to risk having a random in their party looting a class specific weapon or mega rare item that's untradeable on Reboot and worth a bunch on regular servers. So basically, you have to do your social networking and join a community of good players.
Maplestory is very much a long term MMO. You have to get to a threshold level to get into the community, which is really only a matter of time, your progression is measured more in a few months at time than in days or weeks and there is something to do every single day you log in. Most people who say that Pre-Big Bang was better doesn't remember how it used to be and they would quit it if they played it after they played the version that exists now. In summation, ♥♥♥♥ Pre-Big Bang.

Onto the actual game.
The differences between the Korean version and the North American version are staggering, less so with the Japanese and even less with the SEA version. So I'll start off with the easiest.
SEA:
SEA, compared to NA, or Global, used to be different in that they would receive patches ahead of time of NA. To my knowledge this is not true anymore. The only real advantage SEA has is that it has a map called Parkin Lot which makes lvling to 200 even easier. Beyond that, the SEA economy is in absolute shambles and the Cash Shop aspect there is even worse.
JP:
It's been years and years since I've played the JP version or even heard talk of it but from what I recall JP players got 10% more base experience and they have a toad boss that NA doesn't.
KR:
This is the best version of the game. KR Maple does not have Gollux, it does not have Commerci, it does not have ghost exorcist badge, and most importantly it does not have Kannas. So why is this version the best? Because it is the least P2W. The mileage system of KR Maple allows you to buy anything from the Cash Shop, the fact that you can get mileage from bosses, from epic boss spawns, and from frequent events means that you can get pretty much anything you want. You can also buy permanent Cash items either from the Cash Shop or from the fantastic economy. The economy there is wonderful and not crazily inflated. If I remember correctly 1bil mesos there is about $60 USD, which is much much better than any other economy on any other version. The events on this version are frequent and this version gets content the fastest and most frequently as well. The last big difference is that this version has flames on equipment, which is another sort of equipment modifier. I would still not recommend playing this version though unless you can understand Korean and are willing to practice how to type Korean characters. There are a few reasons for this, mainly the English community there is dying and there are only a handful of English speaking players left, you need to know how to type Korean characters to even log into the game and you need them later on to answer anti-botting captchas that randomly pop up. It's fairly simple though to learn, you can learn with a few hours of practice and be fine. The biggest reason I wouldn't recommend this version though is that you have to buy a premade account off of a shady Chinese website for $10.

Maplestory is a pretty fun, mindless game. If you can immerse yourself in the game, you'll have a great time. As is, before you even start the game you have to choose what you'll invest. Do you want to invest time or money into the game? If time, go to Reboot, if not go to any regular server. If you play Korean MS, don't play Reboot.
Now that you've gotten through that, if you're playing on not-KMS (Korean Maplestory) make a Kanna, especially if you're on Reboot, if you're on Reboot playing a Kanna and getting it to at least lvl 150 is mandatory so you can farm gold at a decent rate.
Now that you've gotten your Kanna up to a decent lvl to farm Mesos you can choose what your main class will be. There are a bunch of cool classes like Cadenas and Crossbowmen and what not. Literally any class is viable for end game bosses like Lucid, Damien, Lotus, etc. except for Bishops and Kannas because they DPS way too slowly and bring nothing to boss fights. Ironically Bishops and Kannas are most valuable for grinding lvls and mesos.
Great, so you've brought your main to lvl 200, gotten your Kanna up to acceptable farming rates, and now you're looking for stuff to do inbetween Arcane River dailies, farming for %meso gear and bosses. This is where you go make link and union mules, which gives you a chance to play other classes without feeling like you're wasting time not playing on your main. If you do get to the point where you're making mules for Union get your mules to 141 first for the most convenient Tier of Union and once you have all of them done go back around and get them to 200 for Grand Master Union.

Basically everything you do in this game will eventually get you stronger. Sure you could get even stronger if you spent more money or time, but you'll get there eventually any way. Play it until you get bored, quit for a while then come back and do it again for a bit. You get to scratch your itch of playing a decent, mindless MMO with others and your progression isn't invalidated by new stuff that comes out unless there's a complete overhaul again like Big Bang.

As someone who has played MS since the open beta ended way back when up til a few months ago, and even played the Korean, Japanese and SEA servers at different points in time, I think I can give a prety good overview. A lot of people are blinded by nostalgia when it comes to this game. You may even see people saying that the game was better back when it took 12 hours to lvl up at lvl 73, which is absolutely absurd. The grind to even play your character back then was absolutely ridiculous. The only thing that pre-Big Bang Maplestory had that this version now doesn't is the sense of community and life that was everywhere. People will say that there isn't a sense of community now and that people don't help each other which is untrue. The community just isn't at the low levels, low levels meaning anything below 200. The players are still very friendly and helpful, and will go out of their way to party you if you have quests on the same map, and the Chu~Chu Island PQ at lvl 210 is a bunch of fun. Except for birdsharks. Fuck birdsharks. You've seen me say lvl 200 and 210 previously, so how many levels are there? 250. There are 250 lvls in this game and at no time does the progression ever not give you rewards. 1-140 give you skill points, 141-200 give you hyper skills, 141-250 give you hyper ability points, and every 6 levels after 200 gives you an extra node slot. The first 200 lvls fly by, there are people that can do them in 8 hours and for some it'll take a few weeks. Most notably though is the insane Exp curve. At lvl 200 the Exp required to level goes from about an hour or two to 4 hours or more, especially if you play on Reboot and train in the Arcane maps before getting the 1,5x Arcane force multiplier from the daily quests. And the grind only gets more and more difficult. Previously I mentioned that at lvl 73 pre-Big Bang you could take 10+ hours to level up. Post Big Bang getting from 203-204 takes about as long without the Arcane force multiplier affecting your damage and letting you grind at better mobs. Because as an unfunded character you're going to be grinding at either Scrapyard or the first area of Vanishing Journey for a good 2 or so weeks if you don't miss any Vanishing Journey daily quests. Later on you can expect 2% exp per hour, which, considering the lvl requirements of later areas like Arcana, is going to take you months. As for the social aspect of the game now, joining a good, active guild is necessary. If you don't join a good, active guild, on Reboot especially, your progression past 200 will be absolutely glacial. You need a good guild to do end game bosses with and to get decent equipment, which is doubly true for Commerci PQ because most people aren't going to risk having a random in their party looting a class specific weapon or mega rare item that's untradeable on Reboot and worth a bunch on regular servers. So basically, you have to do your social networking and join a community of good players. Maplestory is very much a long term MMO. You have to get to a threshold level to get into the community, which is really only a matter of time, your progression is measured more in a few months at time than in days or weeks and there is something to do every single day you log in. Most people who say that Pre-Big Bang was better doesn't remember how it used to be and they would quit it if they played it after they played the version that exists now. In summation, fuck Pre-Big Bang. Onto the actual game. The differences between the Korean version and the North American version are staggering, less so with the Japanese and even less with the SEA version. So I'll start off with the easiest. SEA: SEA, compared to NA, or Global, used to be different in that they would receive patches ahead of time of NA. To my knowledge this is not true anymore. The only real advantage SEA has is that it has a map called Parkin Lot which makes lvling to 200 even easier. Beyond that, the SEA economy is in absolute shambles and the Cash Shop aspect there is even worse. JP: It's been years and years since I've played the JP version or even heard talk of it but from what I recall JP players got 10% more base experience and they have a toad boss that NA doesn't. KR: This is the best version of the game. KR Maple does not have Gollux, it does not have Commerci, it does not have ghost exorcist badge, and most importantly it does not have Kannas. So why is this version the best? Because it is the least P2W. The mileage system of KR Maple allows you to buy anything from the Cash Shop, the fact that you can get mileage from bosses, from epic boss spawns, and from frequent events means that you can get pretty much anything you want. You can also buy permanent Cash items either from the Cash Shop or from the fantastic economy. The economy there is wonderful and not crazily inflated. If I remember correctly 1bil mesos there is about $60 USD, which is much much better than any other economy on any other version. The events on this version are frequent and this version gets content the fastest and most frequently as well. The last big difference is that this version has flames on equipment, which is another sort of equipment modifier. I would still not recommend playing this version though unless you can understand Korean and are willing to practice how to type Korean characters. There are a few reasons for this, mainly the English community there is dying and there are only a handful of English speaking players left, you need to know how to type Korean characters to even log into the game and you need them later on to answer anti-botting captchas that randomly pop up. It's fairly simple though to learn, you can learn with a few hours of practice and be fine. The biggest reason I wouldn't recommend this version though is that you have to buy a premade account off of a shady Chinese website for $10. Maplestory is a pretty fun, mindless game. If you can immerse yourself in the game, you'll have a great time. As is, before you even start the game you have to choose what you'll invest. Do you want to invest time or money into the game? If time, go to Reboot, if not go to any regular server. If you play Korean MS, don't play Reboot. Now that you've gotten through that, if you're playing on not-KMS (Korean Maplestory) make a Kanna, especially if you're on Reboot, if you're on Reboot playing a Kanna and getting it to at least lvl 150 is mandatory so you can farm gold at a decent rate. Now that you've gotten your Kanna up to a decent lvl to farm Mesos you can choose what your main class will be. There are a bunch of cool classes like Cadenas and Crossbowmen and what not. Literally any class is viable for end game bosses like Lucid, Damien, Lotus, etc. except for Bishops and Kannas because they DPS way too slowly and bring nothing to boss fights. Ironically Bishops and Kannas are most valuable for grinding lvls and mesos. Great, so you've brought your main to lvl 200, gotten your Kanna up to acceptable farming rates, and now you're looking for stuff to do inbetween Arcane River dailies, farming for %meso gear and bosses. This is where you go make link and union mules, which gives you a chance to play other classes without feeling like you're wasting time not playing on your main. If you do get to the point where you're making mules for Union get your mules to 141 first for the most convenient Tier of Union and once you have all of them done go back around and get them to 200 for Grand Master Union. Basically everything you do in this game will eventually get you stronger. Sure you could get even stronger if you spent more money or time, but you'll get there eventually any way. Play it until you get bored, quit for a while then come back and do it again for a bit. You get to scratch your itch of playing a decent, mindless MMO with others and your progression isn't invalidated by new stuff that comes out unless there's a complete overhaul again like Big Bang.