There's really a lack of consistency in the design of Leopard as a whole...there are about 10 different button/window/color styles going on, and none of them are exclusive to an application. Basically, it makes basic app switching less intuitive, as the different application designs are not individually identifiable or visually the same.
Two other things I don't like: the not-rounded upper corners, which don't result in "more useable space" as you can't use that space anyway. All they result in is a less smooth and flowing look to the menu bar.
I also really don't like the fact that my desktop background has any bearing on the visibility or usability of the dock or the menu bar. I think we learned after the translucent menus in early versions of OS X that wanton translucence is damn annoying.
I can see the interface being workable, but it would have to allow a certain amount of customizability for turning things on or off...which we haven't seen in pretty much any other mac operating system. Maybe we'll be able to set it so stuff doesn't automatically deposit in a stack...I'll never use them, kind of like I never use the contextual menus that come up from the dock. No easier than having a desktop with it all laid out there for you to get to, instead of an awkward pile of them on the dock. Mouse movements upward on the screen are the least accurate because you have to use your arm...part of why I find the dock menus difficult.
Actually, it is supposedly very consistent, at least more so than Tiger. From what I can tell, there are two main window styles now: active and not active. Compare that to Tiger, which has active normal, inactive normal, active brushed metal, inactive brushed metal.
In addition, the menu bar, according to Steve Jobs, adapts itself to what is behind it. I think that this may mean that it changes color and opacity depending on what is behind it.
Also, having the menu bar on top is much better than having it built into the window like Windows, as you don't have to stop the mouse in time to hit the menu. In fact, you can just move the mouse roughly straight up, and keep going, and you'll hit the menu bar. Thus, one direction has no difficulty in aiming, where as built-in menu bars will cause difficulty in BOTH directions.