My main reservation with Steam is the mod communities. How does Steam effect mods and home brew levels?
Do you still download everything, and leave it in a special folder for when you want to play using the mods? That seems like a very strange way to do things, very against the concept of Steam to begin with. Or do the mod & level creators have to send all their content to Valve & Steam to have it distributed with the packages?
The second option probably is the most likely, and with the EULAs that are the norm these days, I wouldn't be surprised that if you want to distribute your content, you have to give valve certain... rights... or else you're SOL. I know Valve has in the past been very supportive to the community, but I smell the potential for abuse. Also, the fact that everything would be piped through the Steam servers would mean that valve had the final say on all maps and mods. That's bad business in my book, because part of the beauty of fan created content is the ability to be free of the developer's control to an extent.
I personally don't see how this can help the mod community, but that's just me.