There's a lot of upsides and downsides to this game, by this current post.

DOWNSIDES:

-This game is graphically demanding, meaning if you wanted to have the best and smoothest experience and a constant 60 FPS, you better have a top-range computer powered by the ghosts of dead Microsoft certified system engineers. Otherwise, once you start using the Flashy and awesome looking skills, you're gonna start lagging. You do, however, get the option to reduce the terrible lag by looking around in the Preferences and turning things off or down to Low.
-Research the classes before you're sure what you want to use throughout the game, you may find certain Main/Sub classes that are too problematic for you.
-Everyone says (And whines) that C9 doesn't lag, but there always will be lag and in this game, its called Connection Lag.
-Customer Support and Service is the WORST, in fact, it makes EA Games look like an active, and helping saint. No one responds to anything, and it even goes far as Webzen banning your account if you complain to them or about them in their own forums website.
-This game is a GRINDER, meaning you go out and spend an entire day farming and looking for that one item that you'll need for something.
-RNGesus.
-There's a Trade Office, meaning you can buy and sell stuff to and from other players out there (Take a look at the prices of what item sells for what before doing so, otherwise you're not going to get much profit, sell too little, you're ripping yourself off, sell too much, no ones ever going to buy it)
-Typical prices in Trade Office are usually in the millions, meaning you better save up and grind as hard and as fast as you can.
-You're going to be a constant customer to the Blacksmith, because there is a repair system and dying halves your equipment's durability.
-Beware turning on Intrusion, at some point, there WILL be a P2W lvl 67 (current Level Cap) with full Perfect grade equipment coming after your head and will humiliate you if you do.
-Beware using Intrusion, if you die while doing so, you don't revive in the dungeon while your target can revive as many times as they want (so long as they have spare Resurrection Scrolls), and if you strike unlucky with a veteran thats busy grinding away and you're not as good as PVP'ing as they are, your ass is automatically handed to you on a silver platter. (Some will openly humiliate you)
-There is a plot.
-There are bound items, meaning you can't store them in a warehouse, they can't be traded and they will take/waste space.
-Gold dropped by normal monsters are rarely worth picking up, completing quests of any kind will ALWAYS give more EXP and money than what you kill.
-Typical skimpy female armor exists here, meaning you get a LOT of fanservices, top and bottom.
-That trailer you saw? Those are called Ultimate Fury skills, and they're the most laggy skills you'll find out there, and they come with a pretty hefty price.
-The enchancing equipment part quickly becomes an all-or-nothing.
-And lastly, this game is dying and becoming a heavily implied Pay-2-Win. Almost every person you'll find in-game is a veteran IRL money-spender, and if you decide to join the P2W crew, Webzen will squeeze every penny out of you.

UPSIDES:

-This game is fun, extremly fun, sometimes addicting and in a few morbid cases, very satisfying. Its the MMORPG version of Dynasty Warriors, minus the politics with NPCs. You're the Big Good who's the only one competent person out there (minus the few temporary NPC allies) that goes out, tosses and paints entire mountains with enemy blood like its another Tuesday to you, and the only "Impossible Bosses and Monsters" are ones scripted to be impossible. Theres even bosses that can be tossed about and thrown everywhere before they can do much to you. You can party with 3 other players to make the massacre even more enjoyable.
-There is a plot, and its decent enough to keep you interested in the "long struggle to save Glenheim".
-There's a lot of classes you can choose, just by the (current) 5 Main classes, each having their own sub classes. And if you want to make different characters, you have plenty of slots to use. (They all share the same Warehouse, however)
-There are even Artisan Occupations which are available for you to choose at the very start of the game. (Research before you choose, though. If you pick an Occupation that won't do much for you, you're not even gonna bother using it and you can't change once you decide. Also, wait a few levels before you decide to spend time crafting, as they can be expensive.)
-RNGesus.
-There's a Trade Office, meaning you can buy and sell stuff to and from other players out there.
-If you don't want to bother using the Trade Office, you can sell or trade them by typing out in the chat and Shift-Rightclick the item you want to sell/trade, theres even a Trade channel.
-Friendly players and guilds do exist in the game, so ask around if you have any questions, they're likely to answer you. But don't be suprised when the guild you joined starts getting silent more often.
-Auspices and Legends boost your character's stats, while Titles give you a sense of impressive accomplishment, the Titles and Legends are found and given by Acheivement Grinding. Auspices are usually gotten through events and buying them.
-If you join a guild and they are at a high enough level, you can buff yourself up with Guild buffs and Guild skills (The buffs are made through Artisan Occupations such as Metalsmithing, Woodworking and Tailoring.
-You can upgrade your gear with the Blacksmith and the Artisan Metalsmithing, making you all the more godly in the dungeons.
-You can craft your own weapons, armor and accessories. Rare grades takes more time but is more worth it than the Magic grades, and the same holds true for Perfect grades over Rare grades.
-Sold something you didn't mean to, including Bound items? You can repurchase them! For double the amount they sold! (So long as you didn't quit the game, or switched characters and you realized quickly what you did)
-A LOT of scenery to go and explore.
-Theres a mentor/student system! As long as the student is below lvl 25 and the mentor is above lvl 40, the student gets DOUBLE the EXP while the mentor gets double guild points and mentor medals, which can be traded for (most of the time) useful goodies.