I set the main default for the website manually at the moment (and it's USD/bitstamp). For everything else there's just algo that decides this based on exchanges and currencies volume. I set what you see when you first enter website manually because otherwise it would be huobi/CNY and not many people want to watch that. When you enter the website second time, you are displayed what you have been watching lately. So newcomers see USD/Bitstamp, others see what they visited lately. But yes, when you click on the currency you see the biggest exchange for given currency first.

I'd like to provide unbiased data, that has nothing to do with my personal opinion. Maybe mtgox should not be default for EUR. *Maybe*. But I'm not sure. I'd rather be coding than keeping track of all exchanges news, especially when it seems really hard to be able to track down facts and not just people opinions. If I would use my personal opinion, btc-e for example would not be on the website anymore based on my support experience with them.