I think your site has A LOT of potential!
I also like the simplicity. It is extremely user-friendly.
I would think about changing up the color scheme. As a designer, we are partial to aesthetically pleasing websites, and your colors are a little uncomfortable to look at. I love the orange, but would rethink the other colors. Maybe check out the Kuler color tool: http://kuler.adobe.com/ It has loads of awesome color schemes.
I love how you have set up the categories - very easy to use.
I'm not a big fan of justified type. Especially on the web. It leaves big gaps like on your "Computer Banners" add on the Latest Freelance Jobs page. I'd left-justify instead.
I would also make all of the links so that they open in a new window. If I'm looking for jobs, I'll probably want to keep coming back to your site. And, it can be hard if I keep losing you every time I click on a link.
Also, just go through and do some design proofing. On the payment page, for example, the spacing is inconsistent, and some titles are underlined and some aren't.
I think you have a great idea here. Your site is very easy to use - and that's the most important thing.