What's wrong is they don't have much competition. There's always some market share available if you put in the legwork and sign up the restaurants OpenTable misses. There's always room for improvement, too.
I just get the feeling there's room for improvement.
I think someone could take the dining reservation concept pretty far as well. A dining reservation + yelp like review site could work well. Design wise, I could see someone doing really exciting things with a site in this sector.