Safari on Windows
Popup animations look crummy. Interestingly, font rendering is the same like on OS X. Plus, the Mac Beta needs a reboot after install, the Windows version does not (my Windows notebook at work required a reboot though). On the Mac the theme still seems to be Brushed Metal.
Performance update: I don't know why Apple states that Safari is the fastest Browser available. Okay, it's still Beta, but this thing is just slooooooooow (on Windows). And website rendering is, well, just not working. In this state Safari on Windows just isn't usable.