My music. I have 109gb of music and if I lost it all I would fucking flip. Anything else worth saving (pictures, videos) are all uploaded onto facebook or photobucket.
Got a lot of music that I would hate to loose. Photos are important, but they are uploaded to Facebook so wouldn't be the end of the world if I lost that folder.
Well, if by important you mean valuable, then it's my Adobe folder. Inside is the Adobe CS4 Master Collection. I don't know what I'd do if I lost that. I can always download music again and get pictures from friends.