Yes, he's brave, and certainly intelligent - but he's also very cunning (modifying Marietta's memory, for example, is a sly move). Although he isn't outrightly ambitious, he does end up Minister for Magic... Also, his relationship with Arthur Weasley (at least in the office) is somewhat cold - I think it would be reasonable to assume this is how it was before the two began to work together within the Order, and that this distance could be interpreted as ambitiousness. Of course, it might just be that the two had no reason to work together, but even ignoring that, he has certainly managed to ingratiate himself with the right people: Fudge, Umbridge and Scrimgeour clearly have a good working relationship with him - he's clearly trusted and respected, and one could certainly argue he's "going places" prior to the Death Eater take-over (what with being chosen for important jobs such as guarding the Muggle Prime Minister).
I don't know, I've just always seen him as a Slytherin. He's what a Slytherin without a strong Pureblood inclination could be. Is that just me though?
