Putting up a well-written, well-presented ad gets people to stop by and have a look at a site, but what makes you stop and actually consider joining?
For me, the first thing would be the overall story of the plot. Is it interesting? Is it something I haven't seen before?
And secondly, I gotta admit, I check-out the Faceclaim section to see if my favorite celeb faces have been claimed or not. It's shallow, I know, but I can't help it.