Ok here a few design and feature position suggestions for your site interface:
I don't know how to best explain this but I guess I can try from the perspective of how I usually trade.
First I want to see the depth table. This is very important to me because even though I don't see dark orders, which btw you should explain somewhere what are the exact mechanics of, I can still roughly anticipate where the market is moving. It's my primary source of information when I daytrade.
2nd I look at recent history. I usually find graphs like these two the most useful:
http://bitcoinity.org/markets?theme=light and
http://mtgoxlive.com/ordersAnd I'd like to also see something like these two when I'm writing my order, meaning both the depth table and a graph like these two should be on the "trade now" page.
3rd when I book orders I want it to be as easy, simple and quick as possible. That means:
-no preview step, or at least the option to turn that off, maybe live calculation of how much a certain quantity would cost at a certain price, or how much a certain quantity would net me at a certain price
-displayed live top bid and lowest ask
-automatic order splitting into 2 portions, one for which I have funds for being active and another which I don't have the funds for in the event that I am trying to buy/sell too much
-easily accessible "my orders" table meaning it should also be part of the "trade now" page so that I can cancel them quickly if I want to
These are some of the thing that I noticed right away and changing them would make your exchange become a lot smoother IMO.
Also I realize you'll have an API available but then I'd have to look to someone for a trading client having to learn how to use it and potentially expose myself to more risks.