Actually, it isn't as difficult as some say!
Firstly,you host the website within a data centre that can handle stupid levels of traffic - like Amazon Web Services
Secondly, you split your requirements into manageable goals - list what it is you like about MT Gox, and what you hate, and clarify what you want to do to make it better.
Me? I want no delay regardless of load. What it looks like is not that important if it makes me money!
If you want an exchange to start with, here is an open source one to use as a template.
http://gitorious.org/intersangoThe first rule of being better than your competition is to start doing something - Think of Google, it has a great search engine, but everything else it does is just ok - but if it hadnt put all that effort into the search engine, the rest of the stuff would never have happened!
Oh - and as a final piece of advice - don't ask for advice - just get on and do it the way you think it should be. If it sux, people will tell you, but if you are trying to build it by committee you will never get anywhere!
Best of luck!