there are lots of things that the 360 opens the door for with XBMC
-DVR/PVR capability (the limiting factor on the 360 was the USB1.1 ports)
-Upscaling DVD playback to 720p and 1080i (they have a little working now but it's not that great because of the lack of ram)
-utilizing the wireless controller board to interface with RF remotes as opposed to IR remotes
-USB based LCD screens (the xbox didn't support hubs which meant it would have to utilize an external controller port making for an ugly solution, the 360 support hubs)
There are many more and that's just with the info we have so far... who knows what other kind of stuff we'll be able to do. Using standard USB ports can you imagine some of the interfacing stuff they might accomplish with ipods or PSPs?
No amount of time thinking about things you CANT do will lead you to a list of things you CAN do.