The most obvious changes, of course, are that the many desktop enhancements that Microsoft first revealed at PDC 2008 but did not fully include in the M3 build are now available. These include the controversial new taskbar, which allows you to confusingly comingle shortcuts for non-running applications and windows with those for running applications and windows. I’ve already written a lengthy and impassioned plea for Microsoft to reconsider this decision–please refer to my article, Windows 7: Simple vs. Easy, for details–but I’m now even more concerned about this feature than I was a month ago. For all the niceties of the new taskbar, this comingling of different functions is a whopper of a mistake, and one that will actively harm most Windows users.

Look, ma: Icons for running applications and non-running shortcuts, comingled in one place. Just like the Mac OS X Dock.
The problem is many-fold. There are now far too many places to launch applications and windows. If you feel that an application or window shortcut is important enough to “pin” to the taskbar –i.e., keep it there permanently–Microsoft actually removes that shortcut from the Start Menu’s Most Recently Used (MRU) list: So even if that item is one of your most recently used shortcuts, it’s now gone from the one place in the UI that’s dedicated to such things. Also, taskbar shortcuts behave non-intuitively and exhibit unique behavior, especially with their Jump Lists, which are themselves non-intuitive. The whole thing is a mess. But it’s a pretty mess, and to be fair to Microsoft, it’s the single major shortcoming in Windows 7.
Aside from the taskbar and its Jump Lists, we also see a fully-functioning Aero Peek (see my Aero Peek Feature Focus) in the Beta: This feature lets you “peek” behind open windows and see the desktop, which could be useful, especially if you use any Windows 7 desktop gadgets (the successor to Vista’s Sidebar gadgets). It’s enabled by mousing-over a new glass panel that’s locked to the lower left of the screen by default.

Aero Peek helps you “peek” under the floating windows to see what’s on the desktop.
Jump Lists also make a very useful appearance in the Start Menu, so as you mouse-over items in the MRU (on the left), you’ll see Jump Lists for appropriate items expand on the right. For example, the shortcut for Microsoft Word displays a list of recently accessed documents. And the shortcut for Windows Live Messenger provides a customized Jump List with Messenger-specific tasks.

Jump Lists work really well in the Start Menu.
Speaking of the Start Menu, this suddenly venerable bit of Windows UI has been modified a bit more since M3 with a new Shut Down button that replaces the previously confusing two buttons that occupied that area. Press the button and Windows will shut down. Or, click the arrow on the right and get a range of options in a weird little pop-up menu.

The Action Center has been tweaked a bit since M3 and gets a new flag icon, which is quite a bit nicer and more easily recognizable than the old lighthouse icon from M3. And Microsoft’s efforts to simplify the tray notification area continue though, as with the usability issues that dog the new taskbar, I’m unsure if hiding things by default is the right approach.
Action Center replaces Windows Security Center and figures to be an important component of Windows 7.
The new Personalization options are really coming along in the Beta as well. Now there are several pre-built desktop themes, which include desktop backgrounds, window colors, sounds, and a screensaver. There are even pre-built themes that utilize the desktop slideshow feature to routinely display different desktop background images. You can, as always, mix and match your own.

Personalization is much simpler and more discoverable in Windows 7.