The main thing I'd like to see Steam on XBox 360 for is for the cross-platform copies of games; being able to buy a game and play in on PS3, then switch to the PC version for adding mods is great.
I generally prefer to just casually play games on my console, as it's nice and simple, no care about system requirements or anything. But console versions are, for some mad reason, usually more expensive. I mean look at Skyrim, the PC version is already more than £10 cheaper than the console version, and it's not even out yet!
When I buy a game, I like to be able to play it on all platforms, particularly if I can only get certain versions (mods) on one platform such as the PC.
I know there are plenty of people who will just say "well you should get a copy for PC to begin with", but it's just not the same no-fuss experience. If I can avoid it, then I'd rather not play games on PCs at all, except for strategy games which still don't really work on controllers. But for modding games it's a necessity, so either Microsoft needs to start allowing modding for XBox 360 games, or allow services like Steam (or produce one of their own) that allows linking of games on different platforms.
The annoying platform lock-in just encourages people to pirate the Windows copy of a game, as who wants to buy another copy of a game they already own, just to install some user-created maps?
It might also allow some proper competition, as currently the XBox Live Marketplace is free to just charge whatever it likes, and never reduce prices, even for unpopular DLC, forcing you either pay the full price for it, or not get it at all, whereas Steam actually has a great track record of reducing prices, offering deals, or even making some DLC free after a while, which makes it a much more consumer friendly experience.