The Engineering Windows 7 blog has a list of thirty six "visible" changes coming in the next version of Windows 7. Here's a quickie list of the ones worth knowing and hotly anticipating or whatever:
Aero Peek—where all the windows on the desktop become transparent except the one you're highlighting—will now work with Alt+Tab. Just pause for a second on the thumbnail you want to scope out. Pretty awesome, since it means less mousing, more peeking.
The Windows key button + [number key] won't just launch a new instance of the corresponding app in the Taskbar, it'll switch to the app if it's already running. And with Internet Explorer, if you keep mashing its number key, it'll cycle through all of its tabs. To launch a new instance of a running program now, hold down shift. Logical, and nice, though as Adam points out, it could be a little simpler still.
Some smaller things are more refined touch capabilities—the keyboard and right-clicking are now multi-touch, for starters. And something I cared about was that the network icon in the system notification tray didn't tell you if your connection wasn't connected to the internet (like it did in Vista), so they've brought that back, hurray.
Windows Media Player's UI is more better! The Now Playing screen has been "cleaned up" (though I'll believe it when I see it), there's more playback support for cameras, a better jump list, and easier access to advance settings.