Lack of backwards compatibility is somewhat of a blow.
I'm guessing there is a second part to the Xbox One--hinted at by the inclusion of both HDMI input and output--that we will probably see around E3.
I'm thinking there will be an add-on (some articles have code-named it "Stingray", and hovering around the $99 mark) that could either act as its own separate 360 console (with separate power source) or daisy chained to the Xbox One. Probably would have its own low-voltage power source, but if connected to the Xbox One powered by HDMI? Probably a self contained hard drive in case it isn't connected to the One, and would allow download of previous Xbox Live Arcade titles via Wifi connection. Not sure it would include a CD-drive, but possible since it would be standalone as well. If connected to a One system would be accessed through a channel menu system similar to Wii U's way of handling Wii titles. Probably would have monthly Live or whatever the new online system will be as a requirement for use.
Probably would be compatible with old and new controllers, but would come packaged with an Xbox One controller to urge people on the fence who might just get it as a standalone system or 360 replacement that if they buy it they can then add to the package and get a Xbox One and already have one of the new controllers.
Would make good business sense since they would capture the early adopters who have a large library of legacy 360 games and PO'ed about lack of backwards compatibility. Probably waiting to unveil to see what comes up next from Sony...
Would be a concession to the used games market for 360 games in the sense that 360 games would still be purchased by system adopters.
IDK. Could be dreaming, but if I was in Hardware Development would be a legit strategy I'd say.