i dunno if this is new to SL or if it was in Leopard since I just upgraded from Tiger, but I like the command-option-eject Sleep shortcut. Also liking the list view for folders in the dock. Ever since Jaguar i think i've been putting my Applications window in the dock and right clicking it to launch non-dock apps, but the right-click on Dock folders into list view was always slow, even in Tiger. Now a folder set to list view in the dock opens really fast. Just started keeping my Home folder in there, thanks to someone's advice on this thread. I used to make an Alias to the home folder on the desktop, but this works much better. Grid view is kinda cool, but not as practical (I guess Fan view was a fad?). The black background list view menus look very nice.
Everything in the Finder in general is animated nicely and smoothly. The Exposé updates are nice, but I don't know if i'll really fall into the habit of using them. Likewise for Spaces, I can't seem to get into using that. Maybe when I get back to work and start having Matlab and lots of spreadsheets open (again I'm coming from 10.4). Also enjoying Time Machine finally. iCal and Mail both sync up with the free online Google equivalents nicely, so I can't really see much of a need for Mobile Me. I got the Mac Box Set so I'm liking iWork a lot. And oh ya, Quicktime X is nice looking, I'm glad Perian still works with it!
Overall, I'd say it's made my 26 month old MacBook Pro feel brand new again. I've always upgraded OS every second major update (to get more bang for the buck, that's all), so this should last me until maybe 10.8 when I might be ready for a new computer finally.