As a 360 owner who's had RRoD I'd put reliability/quality build at the top of the list or a 5 year warranty out of the box.
Other than that I'd go for the following:
- well designed robust pad/controller with rumble
- good/reliable behind the scenes networking for online play/services
- dreamcast compatibility (wishful thinking)

- in built wi-fi
- power pack preferably smaller than a brick
- superb 1st and 3rd party support for games and peripherals
- firefox and email client
- support for any USB keyboard and mouse, hard disk, webcam etc
I guess I'd boil it down to ease and convenience of use, reliability and great games.
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