The serif in Hillary's doesn't convey openness--on the contrary, it reflects the "almost literary" motives more so than Obama's! However, I like the assessment of her choosing her first name to distance herself from the Clinton "dynasty." On the other hand, in my experience, she's better known as Hillary by those who hold an unfavorable view of her.
Obama has the most artistic bumper. His rounded letters actually express the sense of openness lacking in Hillary's stiffness. The "O" symbolizes the rising sun seems overinterpreted to me, but the road paved by the American flag does feel like a new horizon. It slightly reminds me of Bush-Cheney '04, though.
The green in Edwards's is something unqiue--but not necessarily good. His shade blends in with the other colors well, but I think he should have chosen a lighter hue to distinguish better from green's more ill-conceived connotations. The url also seems misplaced, but that's Democratic assymetry.
Giuliani's, not McCain's, looks the most militaristic. Round letters rigidly in place with a patriotic star in the middle, not too much, not too little, it really is a good design. However, while the almost static positioning convey decisiveness, I think the roundness allows for some openness--perhaps a play at Rudy's social liberalism?
McCain's is no winner. Not only are his proportions all wrong, it always seemed to me like he plagiarized this company's logo:
http://www.thomson.com/Romney's is terrible. You would think with all that money he spends on make-up, he could hire some better graphic artists.