Yohodl.com, great questions!
I'm building exchange too and what i've figured out is that:
 - there is a lot of hype around exchange performance. But no one understands what millions of transactions per second means and how many transaction could be made in real life exchange per second.
 - people do not not believe that exchange without FEES could be profitable and people do not trust exchanges without fees!
 - security is very important thing and it's very important how you'll show your ability to protect exchange assets from hacker attacks
 - people behind exchange the #3 factor.

oh and don't forget about marketing. Posting here is only one small little part of the hard marketing work.

Am I right?  Or i've forgot something?