The DSM-IV is a problematic document in many ways. I've dabbled, off and on, with trying to explain the issues... but been quickly overwhelmed each time I try. There are just too many of them.
A recent blog entry by Dr. Sara Hartley over at the Psychiatric Times manages to do a far better job of this than any of my attempts. Oh, I don't agree with her about the DSM-V (which is looking more and more like a train wreck with every press release), but she does a good job of explaining some of the issues with what we have now.