Since certain people cannot seem to discuss concepts without getting personal, I'll attempt it.
The first problem with registration is historically it's been a precursor to confiscation.
It provides government with an easy list when they decide they want to confiscate. It's happened historically in Germany, Australia, California, New York City, Bermuda, Cuba, Greece, Ireland, Jamaica, and Soviet Georgia, etc.
Second problem is, it doesn't help crime in areas where it does exist.
Places like New Zealand, Austrailia, and Canada have said publically their gun registration law did little or nothing to deter gun crime.
Also, in places in America where their is registration, these registries have not helped solve a single crime, because criminals don't register their guns; they usually steal them or get them in other illegal ways.
I picked most of these facts out of the wonderful document at
http://www.gunfacts.info You might want to take a look at it; it's got some wonderful info in it.
EDIT: Noxx, please explain how registration, in your eyes, can help enforce keeping people that "shouldn't" have guns from getting them?
The courts have ruled that prohibited posessors don't have to register their guns, or report them stolen under "mandatory stolen gun reporting periods" because that would violate their right to not incriminate themselves.
As is common knowledge, people that violate the law, err, violate the law. Criminals aren't going to register their guns, fill out a 4473 at a dealer, go through a background check, etc. They will get their weapons illegally off the street. Since it's hard to register a gun during an illegal transaction, I don't see how it can help.
I'll be the first person to say that I'd be for any form of gun control if I felt it would have a decent effect on crime. That's the problem; The war on drugs has done nothing to deter people from getting illegal drugs. Any restriction on guns will have teh same (zero) effect, just like it has on illegal drugs, as well as on Alcohol during prohibition.