While I do feel that the ability to enable/disable individual DLCs is a great addition to Steam, it's not without its problems and The Sims 3 really suffers on that front.

I went to disable/uninstall Pets as it's a big performance killer. Afterwards I was greeted by a nice little error message[i.imgur.com]. Even if I bypass the launcher I get another fatal error[i.imgur.com].

Upon inspection, it seems Steam isn't removing the expansion pack's registry data, thus causing the game to go into an error state. This can be rectified by modifying the registry by deleting the entry for whatever expansion you wish to remove. This should get everything working again, but if you have no idea what you're doing then I suggest you don't touch the registry at all.

If you are planning to go this route, BACK UP YOUR REGISTRY KEY AND SAVES FIRST. If you don't know how to backup a registry entry, Google it... I'm not spoonfeeding those who wish to be lazy. Also note that you are doing this at your own risk and I am not to be held responsible for any damage caused to your game.

To do this on Windows 7, 8/8.1 or 10 (can't speak for other versions of Windows as it's been awhile, sorry. Google's your friend.), press Start, type 'regedit', navigate to 'HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Sims(Steam)', collapse that menu and simply right click and delete everything for the expansion you wish to uninstall. Then start your game and it should be running fine.

As a visual aid, let's just say I uninstalled Seasons and I wanted to remove the registry key. I would simply just right click and delete the entire entry, like in this image.

Unless Valve patches Steam or EA patches TS3 to work with this feature, modifying the registry is the only way to remove an individual expansion.

If you won't wish to play around with the registry, the only way to disable individual expansions is to fully uninstall, disable what you don't want, then reinstall. It's tedious but it's also the safest way to do it, but if you're data capped or on a slow connection then you won't like it.

And that's that. Hopefully Valve will patch the feature to change the registry accordingly if need be. The changes needed to be made to the registry upon installation are actually part of Steam's database, so I'd imagine it shouldn't be too much of an issue to implement.

Hope this helps!