The Weekly Standard's Christopher Caldwell writes an editorial on Dean that I think, while reflecting various biases, is not too far off the mark in many ways. The concluding paragraph:

As for the general election, Republicans seem unaware of how riled up Democratic activists remain, even three years after the 2000 elections. A substantial segment of the party's base has been radicalized to the point where it does not recognize the legitimacy of the Bush presidency. This is a very different thing than mere dislike of a president. It means that Democrats are prepared to fight this election as if they were struggling to overthrow a tyrant. One fears that 2004 could wind up--in its rhetoric and its electoral ethics--as the dirtiest general election campaign in living memory. It is not a condemnation of Dean to say that his rise provides another piece of evidence that this fear is well founded.