Richardson is the best candidate on the Democratic field. He has really solid foreign policy credentials and he is the only Democratic candidate with any significant executive experience. Indeed, he has had a very successful track record as governor of New Mexico, where he balanced the budget while cutting taxes at the same time. His fiscal credentials are solid, and that is a major positive during these times. In fact, the sum of his experiences should make him uniquely qualified to be President.
Richardson should be at the top tier of candidates, given his qualifications, but he has had a hard time distinguishing himself in a very crowded field of candidates. I really suspect that he would be a very good president, but his campaign skills are nearly as strong as Edwards or Obama, who probably wouldn't make good presidents, at least at this point. Indeed, good campaigners many times don't make good presidents.