Most people that enjoy tanking enjoy leadership.

If you are wondering why tanks are counted on to know the fights its because good tanks enjoy knowing the fights. The two go hand in hand. The reason for a shortage of tanks is because there is a shortage of people that want to lead into/initiate an encounter. You have to have the gusto to run in and be sure you wont be one shot, and the guts to take some verbal abuse.

If that scares you then remember it's well within your right to dish it back out when you see the other people doing more subtle mistakes. (Goes back to being a leadership role.) Remember that if you are a tank there are 7 other people that should be supporting you (2 healers primarily, 1 OT/4 DPS or 5DPS). If you feel like they aren't, say so.

YOU ARE THE LEADER. be a bold one!