There are a things that I might change about the UI, but it's nothing that can't be changed with a theme. Oh, and the Flip 3D thing is cool but useless from a productivity point of view. I know Microsoft can't clone Expose lest they be accused of copying, but I hope that there are enough hooks in the APIs that a third party can give us a good use for the rightmost F-keys (or that unused app switcher button on my Logitech MX518).
Performance is acceptable but not amazing with my Geforce 6600 - hopefully that will improve upon release, but I'm prepared to buy a new graphics card if it doesn't get significantly more responsive by then.
By the way, Microsoft is to be applauded for fixing the issue with the start menu on a multi-line taskbar. In XP, the blank area under the start button is not active, requiring extra time and mouse manipulations to bring the pointer back up to the start button after throwing it down into the corner from wherever it originally was on the screen. With Vista, MS has embraced the "mile high (or deep) menu" concept, so this is no longer an issue (besides, the button itself moves down and just looks better on a vertically expanded taskbar than XP's start button did).