The site, which has national aspirations, exists in an already crowded field. For that reason, when I first encountered Yelp early on, I dismissed it as an "also ran." But I must admit the site has traction in San Francisco and, after a redesign a few months ago, looks very good. It's managed to grow by word of mouth and seems to have a very engaged user base.
As the folks at SiliconBeat point out, Yelp has lots of competition: InsiderPages, JudysBook, Backfence, all the YP publisher directory sites, Yahoo! Local, Google Local, Citysearch, etc. (not to mention newspapers). That can only mean consolidation at some point. This is an exciting time for local as the market starts to emerge — as evidenced by the thrice weekly VC calls I'm doing.
Among the "alternative" directories (meaning those not owned by YP publishers), the first one(s) to get to national scale will almost certainly be consolidated/acquired. But by whom? That's an interesting question to contemplate.