One of the new features in Windows 7 that I noticed was the addition of libraries. Windows 7 has introduced a feature that allows me to specify a group of directories as a library.
Let's go with an example. If you have a folder for music in your Documents folder. However, let's say you also download song files through a program like Ares. If you do that, you would typically have to move those files from the Downloads folder over to the Music folder in order for you to listen to the music.
Now, with the addition of Libraries, you can specify a number of folders that contain music, and simply browse to it. For example, under your Start Menu in Windows 7, you will see an area called "Music". This is a link to the library. This will show you music files in all folders that it recognizes as music folders. Right clicking on it and selecting it will allow you to add or remove folders from this list. You can do this for pictures, videos, and documents by default.
You can also create your own custom libraries. For example, I have three locations that I am typically using for school stuff. The locations are my downloads folder (where electronic files I need to check go), my documents folder (where I typically keep some information), and my desktop (where I put stuff I really don't want to forget).
It is also very cool that programs optimized for this (for example, most of the Office suite) can use these libraries as well.
Though you CAN include folders from an external hard drive, you can NOT include locations on a USB drive. That is one negative I have. If someone is technically proficient, it should not be a problem. For example, my USB drive is always assigned to a drive letter of "H". Any non-work USB drive gets "F". I think I understand why they wouldn't allow this - but there should be an option to override it. In my case, there will never be confusion between my main USB drive and others, and I think that is why Microsoft disabled this option. I wouldn't mind if it gave me all sorts of annoying warnings, if it allowed it.
Cool feature, though the USB drive being excluded is a bummer.
Link to more information (with pictures)