Hey, OS X 10.5.2 turns spaces into something useful. See this hint to enable, or just do this:
defaults write com.apple.Dock workspaces-auto-swoosh -bool NO
I haven't used it this way long enough to be sure there aren't a bunch of new maddening edge cases, but what I consider to be the critical usage pattern is now possible when it wasn't before. That is: individual spaces devoted to particular tasks. I can have a couple firefox windows open in one space and a couple different ones open in another space, and from within those spaces cycling through the firefox windows will only hit the local ones. If you cmd-tab to an application that has no window open in your current space, only the menu bar will switch; you won't leave your current space and its windows won't come to you.
I think what I'd really like is a cmd-tab equivalent that's local to the current space. Right now if I have A, B, and C open in space 1, D, E, and F in space two, and G, H, and I in space three, cmd-tab gives me nine applications to sift through regardless of where I am.
The ultimate abstraction ought to allow for each space to feel like a fully independent machine, except when you want otherwise. I think it'll have to be thought out much more comprehensively before we get there. Is there any good reason that screen sharing shouldn't be interface-unified with spaces? I want to switch to space three and be looking at another machine in full-screen mode in its native resolution, then switch to space four and be in parallels running linux, again full screen, then back to one. My new desktop will have 32 GB of ram someday and I'm going to emulate half a dozen operating systems full-time just because. Help me out here, apple.