Humility

Humility.  I always found that it is the most important quality of a programmer.

No one is perfect and I mean no one.  So anyone tries to brag how perfect and how right there are is not perfect and is wrong at least once.  Many people just never realized that.  Some can even be very experienced in a field.

Another subtlety here is to mix up opinionated thought with facts.  I found that when I tried to be truly humble, there are only few laws and theorems in life I can count on.  To take a certain religion, thought, thinking, theory to be granted and correct all the time.  I always find that as a kind of ignorance.  When these symptoms are exhibited in apparently smart people, I found it very tough to accept.

Humility is to accept that the limitation of one's own point of view.  You don't see much in today's programmers.   The worst? Fanboys of programming language X.