It's a good and solid platform for people who have access to decent, unlimited broadband. To those who have metered connections, or unstable ones, some of Steam's can be a downright hassle to deal with. The biggest one I can think of is not letting you play the game until it's fully patched. Those patches can take a while to download on slower connections, and the game may have been running fine.
Mandatory patches is another complaint many have. Some patches break the game for some people, and they want the ability to roll back to an earlier patch so they can still play their game, instead of needing to wait for the next patch. The ability to choose whether or not to update would be nice, too.
While neither issue has been a problem for me(I love Steam, it's incredibly convenient), I can understand others having problems with both.
Another issue I've heard is people complaining about the fact that Steam IS DRM. In my opinion, this is a weaker argument than those above simply because Steam is one of the least restrictive forms of DRM around. You can install and play your games on all the computers you want, you just need to log in.
Another complaint I heard was the fact that you don't "own" any games on Steam, you own a "license" to use that game. Unfortunately, that argument is blown out of the water by any EULA on ANY piece of proprietary software released, which states much the same thing(regardless of how effectively someone could actually enforce that, or the legality of it).
An additional complaint I've heard is the fact that Steam games (usually) require Steam to be running in order to function, and won't if you don't. Now, considering Steam is running on my PC right now, and it's barely using ~30MB of memory, I'd say this is a non-issue as well. If you don't have 30MB of memory to spare, you've got more serious problems than needing to run Steam to play a game.
A side complaint connected to the above one, and also to the internet thing, is Steam's offline mode. Yes, you can play your single-player games in offline mode, and even start Steam without an internet connection, and usually be able to enter offline mode and play. The problem is the "usually". I myself have seen Steam glitch and not be able to enter offline mode, necessitating a reconnect. For me, this hasn't been a problem, but if you're trying to play your games somewhere where you don't have internet access, it's obviously quite a huge problem.
In my opinion, if Steam could make offline mode more stable, and introduce a better patching system, it would make quite a huge difference in the perception of the platform.