Edit: I have taken away my comments comparing my experience in mathematics and math ed departments. I still think there is something true about the kind of character that emerges from working on hard problems which demand a certain kind of rigor, and I also think there is something special (and not known to the general public) about the strong social bonds that exist in the mathematical community, but I don't think that one needs to contrast math and math ed groups to make that point.
My short answer (to which Gil alludes below) was: Persistence and humility.
Though I'm not sure this applies only to mathematicians, but in general to people who work on hard, and perhaps in some ways technical, problems.