I love the fact that the Finder is now metal, simply because you can now click on any edge of a Finder window to drag it around. The sidebar is, I think, really suited to column view, which is my favourite view anyway.
There's been a lot of bad reports about FileVault; on my machine it changed the ownership of everything to root - I'm going to leave this one for a while.
There's more to Expose than they originally said. Pressing tab while in All Applications cycles through focusing on each application.
I haven't really noticed the performance increase on my 12" RevA PB with 640MB, except for the faster boot time.
Font Book and the improved font panel is great, but now it kind of makes me wish for more apps that would use all those ligatures I've found out about thanks to it. There are some haxies that will let you use collections in Carbon apps, but you still can't get those ligatures easily.
Cmd-selecting In and Sent mailboxes lets you see your own posts threaded with the ones you received.
The integration of iPhoto with System Preferences is great, but it only works if your iPhoto Library is in the default location under ~/Pictures.
I look forward to using XCode, too.
All those enhancements add up to a worthwhile upgrade. Expose and the new Finder alone improve the experience by leaps and bounds.