The most impressive quality I've seen in mathematicians (including students) is the capacity to call themselves "confused" until they actually understand completely.
Most of us, myself included, are tempted to say we "understand" as soon as we possibly can, to avoid being shamed. People who successfully learn mathematics admit they are "confused" until they understand what's in the textbook. People who successfully create mathematics have such a finely tuned sense of "confusion" that it may not be until they have created new foundations and concepts that they feel they understand.
Even among mathematicians who project more of a CEO-type, confident persona, it seems that the professors say "I don't understand" more than the students.
It isn't humility, exactly, it's a skill. The ability to continue feeling that something is unclear long after everyone else has decided that everything is wrapped up. You don't have to have a low opinion of your own abilities to have this skill. You just have to have a tolerance for doubt much higher than that of most humans, who like to decide "yes" or "no" as quickly as possible, and simply don't care that much whether they're wrong or right.
I know this, because it's a weakness of mine. I'm probably more tolerant of doubt and sensitive to confusion than the average person, but I am not as good at being confused as a good mathematician.
Although I agree on the whole, it might be worth recalling that 'I don't understand' can be agressive criticism in addition to being humility or a skill. Among many examples of this aspect, I rather like the passage on Kant in Russell's history of western philosophy, where he writes something like: 'I confess to never having understood what is meant by categories.'
In my time in the mathematical community I've formed the subjective impression that it's noticeably less common for mathematicians of the highest caliber to engage in status games than members of the general population do. This impression is consistent with the modesty that comes across in the writings of such mathematicians. I record some relevant quotations below and then discuss interpretations of the situation.
Acknowledgment - I learned of the Hironaka interview quoted below from my colleague Laurens Gunnarsen.
Edited 10/12/10 to remove the first portion of the Hironaka quote which didn't capture the phenomenon that I'm trying to get at here.
In a 2005 Interview for the Notices of the AMS, one of the reasons that Fields Medalist Heisuke Hironaka says
(I'll note in passing that the sense of the "genius" that Hironaka is using here is probably different than the sense of "genius" that Gowers uses in Mathematics: A Very Short Introduction.)
In his review of Haruzo Hida’s p-adic automorphic forms on Shimura varieties the originator of the Langlands program Robert Langlands wrote
For context, it's worthwhile to note that Langlands' own work is used in an essential way in Hida's book.
The 2009 Abel Prize Interview with Mikhail Gromov contains the following questions and answers:
In his MathOverflow self-summary, William Thurston wrote
I interpret the above quotations (and many others by similar such people) to point to a markedly lower than usual interest in status. As JoshuaZ points out, one could instead read the quotations as counter-signaling, but such an interpretation feels like a stretch to me. I doubt that in practice such remarks serve as an effective counter-signal. More to the point, there's a compelling alternate explanation for why one would see lower than usual levels of status signaling among mathematicians of the highest caliber. Gromov hints at this in the aforementioned interview:
In Récoltes et Semailles, Alexander Grothendieck offered a more detailed explanation:
The amount of focus on the subject itself which is required to do mathematical research of the highest caliber is very high. It's plausible that the focuses entailed by vanity and ambition are detrimental to subject matter focus. If this is true (as I strongly suspect to be the case based on my own experience, my observations of others, the remarks of colleagues, and the remarks of eminent figures like Gromov and Grothendieck), aspiring mathematicians would do well to work to curb their ambition and vanity and increase their attraction to mathematics for its own sake.