It's mostly a gut reaction, I admit.
Analytically, I think he's basically Dukakis II, actually: shitty policies/staff preferences wrapped in a package tailor-made for the Republicans to club down. On the outside, he's a Mass Lib. On the inside, he's Bill Clinton -- a pathological liar and social climber living out a JFK fantasy.
About the lying part, does anybody really believe he's against gay marriage, as he says today? That's bullshit -- and it's worse if it's true, of course. But his willingness to lie about it rather than make a stand shows what he'd do to the masses, in the extremely remote event he wins the White House.
As to hard evidence, the fact that he's not being trashed by the DNC/DLC is pretty good. I believe I also read that, as an undergraduate, he was a leader of a Yale movement to _oppose_ draft resistance. And then there's Skull and Bones...