For me the main raison holding back is that I don't like to register to new websites. I like for example Hacker News' Clickpass authentication, because it's easy to use and I don't have to remember another password. But on your site Facebook connect would be even better, because it opens a lot of possibilites.
Another thing is that I don't know how the blog looks like because I can't find a link to it. So a Tour or Help would be nice, or a link to active blogs from the main page.
Other ideas:
Make a contest for newly registered users for a prize of something cool, e.g. iPhone4. Also make a mobile optimized version of the site, or an iPhone app, where the users can find, comment, upload images based on GPS coordinates.