there isnt exactly a "single biggest" reason for me wanting an Xbox One. Its the sum of the features X1 has that differentiate it from what's currently available.
dedicated servers
upload studio
twitch streaming
having access to my profile, saves, and full games at my friends, and family's house with simple facial recognition (the cloud
)
upgraded controller (mainly care about the dpad, but the rumble sounds cool)
Smart match feature (finding a specific game, within a game, while playing another game)
Snap feature (watching tv in one box, while waiting for a loading screen or whatever in another box, with thecoli.com in another box - on the same screen
)