He's articulate, but is he clean?
It is an interesting choice. He seems likeable, and he's flown under the radar for the most part. But that might be because he hasn't taken any strong stances on issues. People only know him because of his acting, not his political career. That might be his greatest asset, however.
Of course, I'll always remember him for the worst line of dialogue in Die Hard 2 -- "Let's pack 'em, stack 'em and rack 'em. Move people!" Who says crap like that?