You haven't answered the primary question anyone asks about a platform that would not use WIN32. That's the question of windows and message pumps. When you know how much anything works with it, including sockets ( :-) ), where are the message pumps gone.
The marketing brochure says Avalon works without message pumps.
This cannot be true (it's marketing after all), otherwise this would also completely and utterly obsolete anything Windows-like. In other terms, no pre-Longhorn app would be able to run on a Longhorn OS.
So the message pump is still there, it's just hidden (like the UI thread in winforms).
That being said, control over the message pump is critical for many apps, so if the new framework doesn't provide any direct/indirect level of control over it, I am afraid that it will be very hard to come up with high performing and well-behaving apps that would at the same time be nice citizens to the overall CPU and system loads.
My 0.5 cent.