The problem is lack of competition. Should a reputable exchange with good customer service (or at least somewhat good customer service) pop up in the US, I'm sure many users would jump ship in a heartbeat... and not just from mt gox but from just about every other exchange. This may be the greatest opportunity for a reputable wall street investment bank or other large, reputable company with the resources to tap into a wide open market where users would flock in droves.
Opening a reputable US exchange would be the business opportunity of a lifetime.
The last two US exchanges, Tradehill and Kraken, both of which boasted about their "professionalism", turned out to be flakes.
It's really pretty simple. To be a Bitcoin exchange or market maker in the US, you should be a SEC registered broker/dealer. There's a lot of "oh, Bitcoin is different and doesn't need to be regulated" talk on here. By now, it should be obvious why broker/dealers are regulated - otherwise, there's a huge temptation to take the money and run, which over half of Bitcoin exchanges have done. It's not clear that it's absolutely necessary under US law to be a broker/dealer to be a Bitcoin dealer. What is clear that if you are an SEC registered broker/dealer, there's no problem. There are even advantages over being registered with the SEC - no need for state-level money transfer registration.
The problem, of course, is that most of the people doing Bitcoin exchanges don't have the training or the money to start a brokerage firm. It's not just paperwork. There are exams, audits, financial requirements, insurance, background checks... That's what you have to do to handle other people's money.
Eventually, if Bitcoin stays around, somebody who is already a broker-dealer will start dealing in it. Then there will be a way to buy and sell Bitcoins without all these delays, excuses, and losses.
Incidentally, as for the Know Your Customer and Anti-Money Laundering requirements, with a real broker, you take care of that when you open an account. You
never get told you can't withdraw your money. If a legit US broker did that, they'd be in big trouble. Some sleazy brokers dealing in Pink Sheets stocks have tried that on occasion. Some of them now have Federal prisoner numbers.