Microsoft focuses on third party support, in my opinion a little too much. It does feel as if they got a few devs to make exclusive games for them and after that focused on third party compleetly, and just being lucky enough to get a few (console) exclusive titles along the way. But that wont help them sell the next XBOX.
On the other hand I don't think that Sony is looking all that hot in the exclusive department right now either. Although they still have enough first party developers to make future exclusive AAA titles to sell the PS4. We'll have to wait and see how it plays out. This gen isnt lost to Microsoft thanks to one hell of a start in 2005-2007.
Nextgen they need to come up with something else. Maybe they have some more tricks but I doubt it, they are putting all their money on Natal and it may work to become the biggest (casual) player on the market. But I prefer more hardcore games, and as a Fable fan Microsoft still has my money but Sony only needs to bring one good exclusive WRPG they'll have it nextgen.