I like:
1) Its community features, I love that I can chat to my friends through steam
2) The fact that I can load non-steam games into steam and do the same
I dislike:
1) The fact that it actually massively disrupts some games. People claim it doesn't, but that's because those people are cretinous jackarses who think that RAM usage and CPU usage equate to the full sum of performance deficit a program can achieve. Games like Mount and Blade that use POP are hugely negatively impacted by Steam to the point where my Steam copy of Mount and Blade Warband dropped to 3 FPS every time it tried to calculate hit impact i.e. every couple of seconds.
2) The fact that they can disable your account and thus basically seize everything you rightfully own. I don't care whether or not they will do it, I care that they can. Would you agree to buy a house if the previous owner stipulated that they could turn up at any time and throw you out if you don't behave as they want you to whilst inside it?
3) The fact that their 'delete local content' button it just an uninstall that wipes all versions of the product from the system, whether or not Steam installed it. That's caused me no end of grief.
4) The fact that you can only view your CD Keys if you've installed the game. That's a nightmare because you need to install the game to view the key to input into the non-Steam version, but then the 'delete local content' deletes the non-Steam version!
5) The fact that they think $1=£1/1 Euro. I think they've finally clocked that a pound and a dollar are not the same thing, but they're still ripping off my EU friends.
Yeah, I don't like Steam, but it's a nice idea and it does have a lot going for it.