I really don't understand how MtGox is still the default exchange. What with all you highly skilled technical enthusiasts devoting so much time an energy to the bitcoin experiment. Oh, wait, that would be actual work and not just a circlejerk. My bad.
I've talked to a few people about making a exchange, apart from the fact that it is difficult to do right the biggest issue I run into is getting USD into such a system.
I live in Australia so obviously I would host the system in the USA or Europe, with the cold wallet stored in Australia. But how do I get USD? I would have to open a USA bank account, and then If I want to accept bank deposits I have to follow the law and verify all the customers and have mechanisms to withdraw and deposit in hundreds of countries. It just seems like a lot of hassle of handling that part, the moving USD in and out.
The exchange is a technical problem that I wouldn't mind dealing with. but the USD is a business problem far out of my capabilities.