Valve is pushing Linux for gaming, and I assume that the Steambox will be a Linux OS. There is only a very small subset of the Steam catalog that has native Linux games, so I am wondering if they are planning on supporting streaming from a PC running Steam to the TV via Steambox? There might still be some latency, but on a local network it is not going to be nearly what it would be streaming over the Internet, such as On Live or Sony's Gaikai.
My biggest question would be how are they going to handle the shared Steam account? What if the wife or kids wants to play a game on the Steambox and I want to play on my PC at the same time? The only way you can do that with the current system on multiple devices is to have one of them in offline mode, which means only one can play an online game at once. What about multiple profiles so that that each user can have their own friends lists and own achievements such as the PS3/360 do on their consoles? Currently there are not any separate profiles on a single account, you need separate Steam accounts, and separate game purchases on each account to do so.