MY PREDICTIONS:
-360 will be backwards compatible with Xbox when you attach the HD add-on. edit: It might not work at first, but a software update later on could enable it on HD equipped systems.
-Halo 2.5 will not only have online co-op and multiplayer over live, it'll have the single player mode with some of the best next-gen visuals of launch. This "port" is not being done by Bungie's main team, but only overlooked by them to ensure quality.
-Add-on will cost $89.00 for the 40 gig version. $140 for the 120 gig.
-DVR and WebTV support will only be possible with the add-on, but not at launch. Sometime down the road, Microsoft will release each through Xbox LIVE as downloads (for a price). Less confusion, easy integration. Also, Microsoft will later introduce its own "ITUNES" type store through Xbox LIVE to sell music, games, movies and more using the same concepts. The "Xpod" could also be introduced at that time and plugged into 360 in the expansion slot. In the beginning it's 100% about games, but later, extra, easy to add features is a certainty. Video Chat and Live Arcade 360 will be introduced as well. edit: You'll be able to put your tivo'd shows and movies onto your video-equipped "Xpod" for extremely easy on the go viewing.
-360 will come with a ~2 gig scratch disk built into *every* unit. Mainly for load times reduction and possibly game saves and music, but not downloaded content.
-360 will have wireless built-in for controllers, customer keyboard/mouse (in the future), and wifi support. With Wifi, the unit will not only let you connect to LIVE, it will likely let you stream music and probably video off your home network. You'll also be able to set up LAN parties much more easily
-hmmm let's see, what else.... Halo 3 for near PS3 launch... 360 and PS3 neck and neck in marketshare... MS partners with or buys one or two pretty big devs. 360 yields very healthy profits for MS (finally!) after 2.5 years on the market (game royalties + microtransactions + LIVE + extra 360 services + ad integration = WOW).