3 GFlops on FFTs. No more gunky low-memory globals. There will be no 68K emulator available in MacOS X for Carbon, Yellow or Java applications. There will presumably be a 68K emulator available within the Blue Box for compatibility with applications that haven't been upgraded. VM will always be on.
Preemption can happen at any time.
What this means to developers is that things like the VisRegion can change between when you get it and when you draw. More on drawing in the Graphics section. No more direct access. You'll need to write a driver. This is a major lose to my mind, but we'll see what develops.