Probably the Surly's no. 1 feature is that it's not a Trek, Specialized, Giant or other major line bike. Also consider the market, many of the newer urban shops owned or staffed by the "cool" folks, cannot get major lines so emphasize, 2nd tier (in brand recognition) or niche brands like Surly, so they immediately have cachet with the cool crowd.

There are many similarities the bike world and the fashion world. Things assume a cool factor within a circle, and ride that for a while until the next cool thing. The bike companies are a bit handicapped here because of the Yogi Berra factor. Nobody buys the most successful bikes because they're not cool anymore.

This isn't to say that there's anything wrong with Surlys, but like you I don't see anything special either.