Documents: You should backup your documents which include your current work contents. And maybe also the temporary folders that are put on the desktop.
Downloads: It's a good idea to backup your downloads
because you might not be able to find the url that they came
Music: If you don't want to disappoint yourself, backup your MP3 downloads which you've paid lots of money for. Because you'll probably be depressed to lose them.
Pictures and Videos: Family pictures and videos are priceless wealth, they are the witness which tell how time taken part in your life. It will not take too much time to backup it up, remind yourself to backup them regularly if you can.