Since I'm a regular Pachter, here is why Gears3/Forza 4/Maybe Halo remake are the only big 1st party titles.
MS knows 3rd party games tend to sell better on their system. They also know a bunch of new/established IPs won't sway a significant amount of customers. Why waste the resources on that, when you can start dumping them into the next xbox. Remember how supportive MS was of the original xbox right before the 360 launched?
With games like LA Noire, Batman, Skyrim, Mass Effect, Saints Row, Battlefield, and another CoD (with map packs on 360 first...bank on it) MS probably feels pretty confident they can sit back and feel their asses grow.