Chicago Boyz has an interesting take on why Sarah Palin attracts such vitriol.
The best explanation for the left’s bizarre Palin obsession is status-anxiety. Status-anxiety occurs when a person believes that their position in a real or imagined social hierarchy is threatened. Leftists react emotionally to Palin because of the threat she poses to their own individual sense of status. All their other arguments are just put forth to rationalize that emotional reaction.
In short, it is not the ideas she puts forth, its that someone like her is significant at all.