I don't like the FAT borders, but I do like the transparent and rounded windows. As a few others have said, it looks like something from the 80's--and while Amiga came from that era, it shouldn't stay in that era.
I want to see more innovative features, and a desktop GUI that attracts attention. I hope the OS gives the ability to easily change the look of everything on it, from different styles of windows to the textures in them and on their borders. I remember some apps that did that for previous versions of the OS, and hopefully the OS will integrate such features.
I'd like to see odd-shaped windows be an option, under style of windows, as well as doing a way with the scrollbars on a window (as an option) in favor of a target field somewhere along the border, allowing for scrolling of content when the mouse pointer is clicked into it and the mouse is slid. And I don't see why the OS couldn't support retractable windows or windows that were animated.
Technology has improved so much since the 80's, we have more memory and faster CPUs, I don't see a problem with having the OS capable of greater eyecandy. A 1980's-looking Desktop GUI is NOT going to attract much attention when on the showroom floor. People will think 'nostalgia' and most will move on.
Aside from the transparent windows, and the rounded ones, I don't see much new here and that sucks. Things could be MUCH better for the GUI. Someone posted in the forums under the OS 4 screenshots topic, a link to a mock-up picture of the desktop that looks much better than these. That picture was closer toward the direction I think Amiga OS should head in..