Here goes. I'm disappointed with what has been said (and not said) by Microsoft and Sony, so I'm going to give Valve a shot. Here's my wishlist for the Steambox.
1.) Full Steam integration. While this could mean "no used games", I'm OK with that. I trust Valve and they have a history of being fair with pricing.
2.) SteamCloud support for all games. Saves stick with your profile and follow you around.
3.) SteamWorks support for mods/new levels. This is something that consoles consistently fail at. User-generated content is excellent on Steam (seriously, check out the levels on Portal 2).
4.) Easy to use, easy to keep up-to-date, and easy/fast to patch games. This is IMPORTANT. I need to be able to pick up a controller and be in a game in less than a minute.
5.) No need to fiddle with drivers, client updates, etc. This should be either a.) done in the background or b.) really fast.
6.) STEAM SALES!!!
7.) Offline mode. If you've paid for a game, and Steam knows it, you should not have any restrictions with how you play that game.
8.) A very Xbox-like controller. The 360 controller has become the de-facto PC controller, either support it directly or copy it unapologetically.
9.) A good matchmaking system, similar to Xbox.
10.) Cross-game voice chat.
Any others I'm missing? Sound off below.