For one thing, a registry is still full of unenforceable holes. There are currently hundreds of millions of privately-owned guns in this country and no reasonable way to ensure that they're all registered. I suspect that if a registration requirement were passed, its compliance rate would be surprisingly low much the same as when Canada attempted a long gun registry. Similarly, even if compliance was high, it would be relatively easy for an unscrupulous person to bypass it. If, for example, you wanted to sell a gun to a known prohibited person, all you'd have to do is report the gun stolen after the sale. Afterall, the buyer would already be in posession of illegal goods (an unregistered gun) so what difference would it make if the gun was reported stolen?
More importantly, however, registration opens the door to intimidation of lawful gun owners and abuse of gov't power. The whole fiasco with The Journal News in New York wouldn't have been possible without NY's registration requirement. Likewise, what's to stop an overly zealous or unscrupulous gov't official from conducting ulawful gun confiscations during an emergency such as we saw in New Orleans after Hurricane Katrina in 2005? Sure, you'd have recourse through the courts afterward, but that would be of little comfort then and there when the looters are roaming the streets.
Even if not overtly illegal, there are many ways in which gov't officials can harass people if they want to. For example, if you own a firearm that one of your local LEO's doesn't like, what's to stop him/her for pulling you over for whatever minor infraction that he/she can think of/fabricate at every opportunity? Likewise, what would stop an anti-gun appointee in your state or federal tax agency from automatically auditing the income taxes of everyone known to own a certain type of firearm?
The main issue is that a registry of firearms and/or firearm owners would only affect the law-abiding. By definition, a prohibited person cannot legally possess a firearm regardless of how it was obtained. A criminal is not going to register the gun that he's not supposed to have in the first place (and even if he did, we'd be running into 5th Amendment issues), so the only people whose guns the gov't will be able to keep track of are the people who were never the problem to begin with. The crux of the argument against gun/owner registration is that it places unnecessary burden and/or expense on the law abiding while doing little or nothing of substance about the actual problem of violent crime.
Smith, and Wesson, and Me. -H. Callahan
Well waddaya know, one buwwet weft! -E. Fudd
All bad precedents begin as justifiable measures. -J. Caesar