Little things. For one, I don't like the new 3d dock. Or the semi-transparent menu bar. And the terminal program, which I use a lot, seems jerky when you hold down a key and let it auto-repeat. Didn't do that in Tiger. Also, in general, I gather that it has higher resource requirements compared to Tiger. If I could fix the dock, menu bar, and that weird thing w/ Terminal, then I'd be mostly satisfied with it.

Oh, also, I wish they'd have shipped a 1.6 JRE with Leopard. And updated Xcode to include a newer version of gcc.