I read most of the comments in this thread before I viewed the photos, and I was expecting the worst, but it's not actually that bad. A unified look is what everyone has been wanting for a long time, and this is a beta.
In some cases I like the gray look they are using now. It's actually nicer than brushed metal, however, the problem with it is that it clashes with the white-colored content areas in windows (examples are top of spotlight compared to bottom of spotlight, system prefs, open window and to a certain extent photo booth).
In the finder it looks alright because there really isn't a clash of themes.
I really liked the theme of mail right now, so I hope that they will include 2 unified looks in Leopard when it ships, this gray one, and a white one too. Allowing users to change the whole theme would be a pretty good step to take I reckon, especially seeing both themes are very nice, but you really shouldn't use both at the same time.
On the issue of the straight corners, at first I was skeptical, but it will actually create a nicer look when you put 2 windows next to each other (instead of right now where you have a few pixels showing through from behind around the corners).
Not sure if anyone noticed this...
In the Finder window, look at the back/forward buttons. The arrows are closer together than usual (open Finder and compare) and I don't like it. They're off-center.
Ooh your right, that is bad... I'd imagine they'll def want to fix that.
Edit: Notice how the spotlight back/forward buttons aren't suffering from the same issue and the view change is actually in a different order with a different button at the end. You'd of thought they'd keep them the same.