I was surprised by Spaces. I'm not a big virtual desktop fan. Especially with Exposé and the way OSX handles app switching via the keyboard, I never felt that it was needed. Well, I stand corrected. Spaces is an incredible implementation of the virtual desktop concept. Even better is the integration with Exposé and hot-corners. For example- I've got a Finder window open with a pile of MP3s. In another virtual desktop, buried under six windows I've got iTunes. I want to import those MP3s into iTunes. My process? Select 'em, drag the bunch to the (in my case) top left corner. Boom! Spaces pops up. I'm now looking at all my virtual desktops. Slam the mouse over to the top right corner, and swish- all the windows in all the desktops align themselves so I can see everything. Hold my mouse over iTunes- the screen flickers and then zoom- I'm looking at iTunes as my frontmost app. Two fast, low precision mouse movements give me quick access to everything running on my system. Nice. Oh, the eyecandy is good too.
Stacks is getting ripped on a little bit for being... well... useless. I agree, but- there's a lot of potential there. Right now, stacks are little folders that serve as pop-out drawers. Click them, and all the files come flying out in eye-candy mode. It's nice and all- but not that useful. But I don't see any reason why the Stacks have to be limited to files and folders. Why not emails? Or RSS items. Or CPU monitors. Or bookmarks. You get the idea. Once third-party devs find out how to hack Stacks, they'll rock. Mark my words.