UAC's nag-a-riffic behavior can now be dialed down a great deal more easily.
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Apart from Product Activation, no other feature in Windows has generated as much ire as user account control. The basic premise was sound -- do not let programs run with administrative privileges unless the user explicitly allows it -- but the implementation turned out to be a huge bother. Some people tweaked UAC with Registry changes to make it more palatable; others turned it off entirely (and thus defeated the whole purpose).

In 7, UAC hasn't gone away -- it's just been sent to obedience school. It's now both far better mannered and manageable. By default, any changes you make to system settings through the Control Panel (or other things that are part and parcel of Windows itself) go through without needing UAC approval. Launching programs that must run as admin still need to be approved, although you can elect to allow such programs to launch "silently" (not a good idea).

And yes, it's also possible to turn UAC off entirely -- although if you do this you're essentially back in Windows XP land, where one piece of malware launched as admin without you knowing about it can ruin your whole day. Thanks, but no thanks.

Resource Monitor: Task Manager Reloaded

For years the Task Manager has been the most common way Windows users have peeked under the hood to see what's running (or, sometimes, what's not running). It's been augmented or even replaced entirely with third-party programs -- Sysinternals' excellent Process Explorer is one of the best -- but with Windows 7, Microsoft has bundled something that could be called Task Manager Reloaded: the Resource Monitor.

Fire up Resource Monitor and you're greeted with four tabs (plus an overview tab) that provide you with a running analysis of the four most common system resources: CPU usage, memory consumption, disk activity, and network throughput. This allows for a fast at-a-glance way to find out what's eating up most of any given commodity.

Example: If your system's suddenly exhibiting a great deal of disk "chatter," click on Disk and sort Disk Activity by Total Bytes/Sec to see what comes up at the top of the list. If you see something that's gobbling up a lot of disk throughput, there's your culprit. Note that if said program is listed as having an I/O Priority of "Low," that program should gracefully allow other apps to use the disk as needed, so the fact that there's disk activity alone is not a sign that your system is choking. Resource Monitor makes this kind of debugging relatively easy.

On the same note, if the Disk Queue column for a given disk in the Storage pane shows a high value, it means you have too many programs competing for what's on that disk. You might want to move things around -- place swap files on another drive, for instance.

Another thing you can do with Resource Monitor is analyze programs that seem to be stuck to find out what's holding them up. Right-click on the name of any process and select Analyze Wait Chain, and you'll see a breakdown of what that program is waiting for. It's a good way to find out if a program is hanging because it's waiting for something else to finish, and not because it is itself stuck.