I think Apple are tacking on too many 'modes' onto OSX for my liking. Expose, Dashboard, Front Row, and now Time Machine and Spaces; all of which are outside the normal Finder experience.
I think Expose and Spaces could work together well though, either Expose will effectively ignore Spaces (i.e. hitting F9 will show all windows in all Spaces), or Apple will add another Expose keystroke for Spaces, so you'll have (for example):
F8 - Show all windows
F9 - Show current Space windows
F10 - Show current application windows
F11 - Show desktop
It does still seem to be complicating things quite a bit.
Core Animation probably means we'll be seeing a lot more 3rd party apps with the look and feel of Front Row or Time Machine. It's could be a big differentiating factor between Mac and Windows apps, but it could also lead
to very, very tacky apps coming out.
Also, many of the announcements made or hinted at were application rather than OS features. Pretty surprising too that Xcode 3 didn't make it into the WWDC keynote.