-You should change the "How much?" to "Sell Bitcoins" on homepage as its one of the first things you notice.
-A little banner or a paragraph below the logo and top bar containing some information would be good IMO, or maybe you're going for a minimalistic style. However a slightly longer text about your business somewhere would give a certain feel of trust and transparency (which are both really important).
Here's a pic of what I mean with the banner/paragraph:
-Adding a referral program would give you more publicity as its tempting to share the links around when you know you can profit from it. Even if its a slight commission.
-Advertise, create campaigns (e.g. chance to win something/ less fees with a code) that attract people.
-A new and more prominent logo would be great as it would help creating a brand, the users are more likely to remember the site. It should match with the overall style, so something simplistic, but not as simplistic as it is now.
-You should put the "current price" somewhere else (my suggestion below in pic) and move the logo where it was. A catchphrase along the lines "Lightning speed (or fast) exchange" (if you're going to keep a lightning bolt as your logo) would be nice too.
(Sorry for my extremely poor image-editing skills, I will try to make better examples and update this soon.)
-Add a livechat or something similar so the customers can easily get help.
-Get trust certificates (e.g. Verisign/SSL like already been said)
-Aim for good and fast customer service as you are looking for long-term users
-Get a mobile version/app designed
The service page is pretty much perfect, I wouldn't change or add anything myself. Pricing seems really easonable. If I come up with something I'll update this post. Overally the site seems really good.
My BTC address if I somehow happen to win:
16rVxgVAPvGWerpTu5TEt4WsBY29JSdkTp