Non MS platforms: I have no idea. Which video do they say that? Do they actually say 'non MS platforms' or do they say 'cross platform'. Right now it seems that 'cross platform' means PC, Xbox & Xbox360

Managed: If you are asking about the 'dreaded performance difference', thenI have to assume you are basing that on heresay and bad forum posts. Yes its slowly, but not by much. I'm not claiming you could write the top 50 AAA titles using it since they are perf tuned beyond what most programmers could do, but the rest of the games out there you probably wouldn't notice other than the fact they took less time to develop and crashed less often.

D3D10: I've seen nothing to say it will be supported on xbox360. Source? Statew of SDK - its called a beta becuase its not finished. There is no hardware yet so anything you do until then is purely investigative and a learning experience.

Proprietry Software: I'm not really sure what you are asking here. XNA framework should be just as good , if not better, than Managed DirectX today. Sure its proprietry but MS have product to sell.

Jave vs C#: I have no idea on the perf.