For me its all the totally new IPs like Mass Efect, Gears of War, Bioshock and a huge amount of others. As much as I like Mario and Zelda, after the 67th Mario game I tend to want a little different things sometimes.
Runner up to that would have to be Xbox LIVE. Greatest online EVER.
Whats your favorite part of the 360?
GOTY Contestants this year: Dead Space 2, Dark Souls, Tales of Graces f. Everything else can suck it.