For one, Areo Glass is amazing. I have to say that once enabled, you won't want to disable it. If you don't have a computer that can support Areo Glass, then update or don't bother with Vista, as without Glass, Vista is nothing.
For example, here's winkey+tab:

It doesn't look like much, or a waste of resources and slow, but it is neither. The abiltiy to see in real time what each app is before you take to tabbing to it is amazing. This is updated in real time as well, so you can even watch two applications update (watching a google video while keeping an eye on IRC for example).
It is also amazingly fast. As far as one is able to work out, the processing of the GUI is handed down to the grapics card, and even my old 9600 Pro was more than enough power to make this smooth.
Speaking of ATi, for the moment don't try use the offical ATi Vista RC driver. It doesn't work, (at least not on my 9600 or my friends x1600). If you happen to install it, go into Vista's safemode (F8 on boot), and uninstall or revert the driver. Just make sure that Vista doesn't see the ATi driver still. You can then boot back into Vista and use your OS.
Also, if you install or run an application that doesn't support Areo Glass (of note, Java runtime is one), then the OS drops out of Glass. So far the following apps haven't supported it:
- 3D Mark (01 and 03)
- Java runtime
If you install the Java runtime for Firefox, anytime you run firefox, Glass is disabled. Only by uninstalling Java will Areo work.
Back to Vista!