MS started with a lot of original IP's in a lot of genre's this gen but not all them worked (most didn't)
Kameo, Mass Effect, Blue Dragon, Gears of War, Lost Odyssey, Ninety Nine Nights, Alan Wake, Crackdown, Infinite Undiscovery and Viva Pinata.
I think it is a little unfair to claim that they didn't try for variety. They failed but they tried.
Yes, they have been a little dry of late but I've had the suspicion for a while now that they are regrouping for next gen. They've had Turn 10 for some time, and now they've built up the promising 343i. The moves by these individuals add to this suspicion. Sure, they are only 3 or so people but they are key people on big, high quality games. Of course, this could all be wishful thinking but time will tell.