I think the most important thing for me is some kind of assurance that they can't just flip the servers off and kill my entire games library overnight. While it's unlikely that MS will ever turn off their servers, it would not be out of imagination for them to turn off the XB1 servers to incentivise people to move on to the NEXT Xbox system beyond this generation.
They did, afterall, eventually stop supporting Windows 98, 2000, and XP and will do the same for Vista and 7 in the future.
When will support stop for the XB1?