Now that he's dropped out of the race, it hardly seems worthwhile to comment on Wesley Clark, but I will anyway. I always thought that Clark's biggest problem was that he wasn't a real Democrat, but he played one on TV. Thus whenever Clark was asked about his positions, he always came across as a caricature of a Democrat. The best example is when he was asked about abortion and said that "Life begins with the choice of the mother." This is not a position that can be defended on either scientific or religious grounds, the only ones which are truly convincing in this debate, so most pro-abortion rights politicians don't talk about life at all, and instead talk about the right of a woman to decide what to do with her own body. The best Democratic politicians can talk about how abortion is not a good thing (very, very few people believe that it is), but it must be protected since the alternatives to abortion rights (unsafe illegal abortions, unwanted and abused children) are worse.
There are other examples, but ultimately what did Clark in was that he wasn't a Democrat, he was just pretending to be one.