It was a real tie for Spaces and Quicklook for me... Spaces won out. As a unix sysadmin it is nice to have this functionality back. I used to use multiple desktops all the time on X-Windows systems, and I never really found the current Mac OSX 3rd party Virtual Desktops apps very satisfying.

Quicklook = just hit space, gotta love it.

But what really made me go WOW was the translucent menus (no NOT the menu bar, the menus). Damn, that's beautiful. Translucent, yet blurs out the lower layer enough so it doesn't interfere. Just stunning!

Oh, and if I ever get back to my mac working, or when I get a chance to try sharing with mom, that will probably rank highly.

that's funny, the blurring is the thing I absolutely hate the most about Leopard.
First you put a strain on the system by making something transparent, than you remove the only conceivable function of said transparency, namely to be able to distinguish what's underneath.
The blurry mess takes away from the cleanness of the interface as well, imo. (And the fact that they stole this idea straight from Vista doesn't help either)