I don't think anyone can give you a really good answer to the first question. If you can, contact your interviewer and see if he or she is willing to talk to you about why you might have been waitlisted. Then, I'd see what you can do to improve your chances based on what he says. The reason you were waitlisted will give you insight into the school's concerns.
A post-interview "hold" typically means that the school doesn't know what to do with you right now. They are on the fence in some way shape or form. If you are really interested in the school, write a letter expressing your interest.
Good luck.