His best trait is his devotion to his people and city. Unfortunately this is also his downfall. There is a sense, in Tolkien, that what we most want is the thing that is most dangerous to us. Frodo originally wants only to live in the Shire; had he remained there after all that had happened to him, who knows what torment he might have gone through. We don't really need to spell out how bad Gollum's desire for the Ring was for him. In Silmarillion Feanor's desire for the silmaril wreaks havoc on Middle-Earth. Boromir is another example of this principle. He wants to save his city, so he doesn't really fight the call of the Ring, I think. It is playing on his greatest weakness.
Boromir has this all-consuming zeal for Gondor that he believes his birthright confers on him. No one can possibly understand the gravity of his responsibility to Gondor.
There is this haunting desperation in his desire to save it, as though some part of him wonders if he's up to the task.