One of my favorite Mac tips, which I shared on TUAW ages ago, won't be making the transition to Leopard. According to David Pogue you can no longer drag your Mac's Hard Disk into the Dock. I'm not sure why Apple thinks that Stacks and a speedier Spotlight can replace the ease of use that this trick offers up, but I'm hoping that it will make a triumphant return in 10.5.1.
David also highlights some other features that didn't make it into his full Leopard review.
Update: Several commenters say Pogue is flat out wrong about this. I don't have Leopard yet so I can't say for sure who is right, but it would be silly of Apple to have removed this feature.
Update 2: It would seem that Pogue was right and I just misread what he wrote. You can drag a hard disk into the Dock, but right clicking on it (or any folder) won't bring up that lovely menu you see to the right. Everything turns into a Stack in the Dock, which is decidedly less helpful if you ask me.