If you are a user coming from Windows XP (or any other older version of Windows) and you commonly use your mouse scroll button, you're in for a nasty surprise in Windows 7: mouse scroll behavior is changed. Now the mouse scrolling only works in an active window. Even worse, two-pane windows (like Explorer) force you to make the pane active before mouse scrolling works.
So what, you may ask? An extra click, big deal. Well, apart from the fact that it's unexpected behavior, which is annoying to get used to, it really adds to the number of clicks back and forth necessary to do something simple. Save a few seconds here and there, before long it adds up!
There is no fix so far from Microsoft, but a third party tool exists: WizMouse. It's freeware, and works very nicely to restore this basic behavior.