I was recently asked a similar question this weekend. Apparently, Hillary does not score high on the single women Democrats list.
Behavior analysis is "my thing" (and a large part of my job).
Hillary is my current preference, but I can understand why people do not like her. I grew up in NY and was pissed as hell when she ran for Senate. It angered me that someone from Arkansas could pick any state to run for office -- just by playing the "first lady" card. After time, she proved herself and actually won me over.
The problem with Hillary is that she is not a great reflection of the typical woman. She has been playing with the big boys in a man's world for too long and is missing a feminine softness. She has a hardness to her, of which I respect. Yet, she never seems to struggle with any of the tribulations facing the every day woman. This makes it hard for many woman to relate to Hillary. She is in a way robot-like; unplagued by menopause, weight issues etc...
Of course people want their leader to be strong and powerful. But, we also want the human aspect. We want our leader to be charismatic and likeable. Stereotype or not, women are connectors and relate more to others on an emotional level. (Disclaimer: I have a Psychology degree, however it does not make me an expert and anyone is free to argue me on this point or any that I have made thus far).
I think Hillary is missing the emotional appeal. Which is not her strong suit and might not be the best tactic for her. Her strength and almost "asexual" appeal is one reason why many men find her to be a credible candidate. (Afterall, Bill is the charismatic one right?)
This is just one thought and viewpoint. Hope it helps.