I think an understated part of it just the gameplay alone.
I believe there is huge under-served gaming audience out there that couldn't care less about playing the same types of games over and over again, which most mainstream releases are. FTL is just a straight-forward good concept for a game, unique, and I think many gamers who just watch any videos or a few screenshots can see the appeal of it right away.
I found out about it from TB. I wish he would have covered it while it was in kickstarter though, I would have been so happy if I could have been in the beta.