Eh MS hasn't ported exclusives in ages. The last one was Fable 3. EDIT: Forgot Alan Wake.
Fable 2, Crackdown, Crackdown 2, Banjo-Kazooie N&B, All Kinect exclusives, All 360 Halos, all Forzas, PGR3+4, Saints Row, Dead Rising, Lost Odyssey, Blue Dragon, Infinite Undiscovery, Magna Carta 2, Tales of Vesperia (not counting the Japan exclusive PS3 version), Gears 2+3 (and likely Judgement), Burnout Revenge etc.
Also there are a lot of games that don't see a PC release at all and some that do, get a port much later like GTA (or any of the MS ports).