It gotta be cheap... say $60, digital download should be $20. If they do that, hell with physical discs. Oh, also maybe some coupons for CE/LE add-ons, that would be totally awesome!
Oh, no need to be available for "game sharing," but i should be able to download the games i purchased unlimited times. The purchases should be tied to my account, so that if my console is replaced, i should be able to "attach" the games to the new console w/o so much headaches. Game saves should not be tied to account though, so that you can e-mail them to other players. However, individual achievements/trophies are attached to your account. And yes, hdd should be upgradable, not some primary/first party crap like the Xbox 360's hard drive. That implies backing up and restore using external hdds.
If they can do that, that would be awesomely awesome... Personally, i hate to put a disc into the console. Why can't i just press a button and the entire personal library is in a menu eh?