The new taskbar
Windows 7's taskbar is similar to the Mac OS X Dock --- Microsoft has shown good taste in stealing from the best. Like the Dock, It does double-duty as an application launcher and task switcher, and it even improves on the Dock. If you've got an application open with multiple windows, the application's icon changes to show multiple icons stacked against one another. Hover your mouse over the icon, and you'll see thumbnails of all open windows. Click any to switch to it. You can see it in action, below.
Windows 7 also has a nifty feature that makes the taskbar even more useful, called Jump Lists. Right-click an application's icon in the taskbar and a menu appears of actions associated with that application. The the list varies according to the application. So example, when you right-click Microsoft Word, you'll see a list of recently opened files, but when you click Internet Explorer, you'll see a list of your most frequently visited sites. You can see a Jump List in action, below.
Aero Peek is the slickest piece of eye candy Microsoft has created in some time. Mouse over the Aero Peek button on the right side of the taskbar and all of your open windows disappear. However, your open windows don't entirely disappear -- you also see the outlines of each. You can see straight through then to your desktop, including to gadgets and icons sitting there.
It's just plain faster
I run Windows 7 on a Dell Inspiron E1505 notebook with 1GB of RAM and a 1.83 GHz Core Duo processor, certainly nobody's idea of a speed demon. But the operating system is quite zippy, certainly faster than Vista. Applets and dialog boxes appear without a delay, unlike Vista.
UAC is now usable
User Account Control (UAC) was certainly Windows Vista's biggest annoyance, popping up constantly, and interrupting seemingly innocuous tasks like changing your system date and time. No longer. In Windows 7, UAC gets out of your way. It strikes the right balance between security and usability. Far fewer prompts appear, and the ones that do appear pop up only for good reason.
Better document organization
Windows 7 introduces a new way of organizing files called Libraries, which replaces the old Documents organization. It's more than just a name change -- you can add virtual locations to the Library. So you can place network folders there, folders from another drive there, and so on. They'll still live in their original locations, but will also be accessible from one location -- Libraries. You can see it below. There's one drawback to virtual locations in Libraries, though: You can't save files to the virtual folders, and have to save in their original locations.
Search has also been improved. It searches through your Libraries, which means that you'll be able to search any virtual folders there. So you'll be able to search across your network. Search also displays long snippets for each result, so that you can easily locate the file you want among many search results. And you can easily customize and filter your searches using file name, author, file type and file size. You can also add tags from in Windows Explorer to individual files, and sort by them.