I'd work on scanability on the home page. I'd make the unordered lists larger, with a bolder font to draw my attention to them. I'm having trouble coming up with good copy for those areas, but you should see if you can reduce the text. There's that rule from "Don't Make me Think" that goes "Get rid of half the words on each page, then get rid of half of what's left."
I think it would be nice to have some more categories and/or subcategories. I see that I can subscribe to requests for "Technology", but I know nothing about hardware, or phones. It would be nice to only get requests for "Web Design" or "Web Sites".
The UI is pretty plain at the moment. Nothing really wrong with that, but it looks a certain professional polish.
In terms of a niche, I haven't spent much time thinking about it, but when it comes down to it, this is essentially a general anonymous message board like Craigslist, so I'm sure there are tons of niche applications. I'd love to hear more ideas from folks.