Well, nothing really compares to Awesome's tagging system (and I say that after only managing to survive a few days in Awesome, I need a bit more structure by default).

I'm not really sure why, but Spaces just feels inferior to basically every multiple desktop implementation I've used on Linux. There's just not enough of a UI for it, and it's clunky to use. On Linux using multiple desktops is smooth and natural. On OS X, it just feels weird.