The drop shadows are too big for my taste, and the new contrast between non-selected and selected windows is too big as well. I much prefer the new GUI to the old, though, and it seems Leopard is ny new main OS (haven't booted Tiger since I installed Leopard).

Edit: Uhm, beside the GUI. Well, the last sentence above says quite a bit IMO.
Stability is no doubt worse than Tiger at the moment, but it's a leaked beta, so...
When typing in Spotlight (I use it to search folder structures etc), the search box loses focus after a few letters, means you have to click it again. Annoying. Also, it doesn't save WHERE to search, I always want to search beneath the current folder but it always switches back to "Computer", Tiger didnt' do this.
The Airport menu extra causes massive CPU load, so you have to turn it off... Only you can't connect to a network without it, it's broken in the network prefpane.
I can't click the receive file-button in Adium (running 1.1b5), dunno if that's due to Adium or Leopard.
System Preferences sometimes locks up using 100% CPU. When I tried to change all .avi files to open using VLC, Finder just closed the window before I was done.
Quite a list! But I'm still not going back to Tiger, that should say something, shouldn't it?

I can live with the above for the moment. Spotlight I don't use a lot. Airport is easy, just enable the menu extra, join a network and disable it again. I set Adium to auto-accept from my contact list, and VLC solved itself once I realized that it DID work despite the error message.