Task Manager
The Processes tab now shows the command-line and a description for each process. You can also right-click on a process and view its properties or open the executable's folder. These improvements make it real easy to figure out what each process is for (especially for identifying services associated with svchost.exe processes), and for rooting out spyware and gratuitous resource hogs.
The new Services tab is much more convenient than the Services applet. You can navigate directly to the process associated with a service by right-clicking on the service and selecting "Go to process".
Resource Monitor
You can see which processes are consuming the most CPU, memory, network bandwidth, and disk access. This takes the guesswork out of figuring out who is hogging up the network bandwidth.
Ubiquitous and fast search
There's a search box in the Start Menu, Open File dialogs, IE 7, and Windows Explorer. Definitely speeds things up.
Oooohhh, pretty!
The Windows Aero glass interface looks very nice. My nontechie wife was won over by the new photo screensaver.
Automatic offline file synchronization
Haven't tried this yet.
Windows Explorer
Tasks are now placed in a dedicated toolbar instead of the XP way where a task panel fought with the folders panel for screen real estate.
The new drop down arrows in the address bar make it very easy to navigate your directory hierarchy, especially when you need to backtrack to a neighboring subdirectory (I find that I do this a lot).
Other new features are the Open Command Prompt Here and Restore Previous Version right mouse menu items. Vista automatically stores previous versions of files and folders. It must be doing something fancy under the covers to not store the entire contents of the file/folder (perhaps it's using the differencing algorithms used by offline file synchronization).
I like the level of detail control for listing files (CTRL+mouse wheel).
The only thing I don't like is that the default folder that you start in is your profile folder which is totally useless. Just getting to the C:\ drive takes several clicks and scrolling. Much more painful than with XP.