What does it take to be a tank?

I mean besides a group of people willing to put their lives in my hands? Do I need to know all the fights or can I just wing it?

I've got a character geared and ready to tank dungeons but I've hesitated because in my experience with other MMOs I've learned players can be pretty impatient with tanks who are trying to learn the ropes. Unkind, even.

EDIT: I'm not asking about class/build/gear.