Starting to tank is scary. During BC I leveled from 40 through 69 as a Balance Druid, and then discovered that a tank couldn't be pugged for love or money once you hit the later portion of Outland on my realm. I'd picked up some feral gear along the way and decided to give tanking a shot, but then spent my first afternoon in the trenches being a nervous wreck after losing aggro to impatient DPS, aggroing extra mobs by charging into pulls instead of LOSing them, and taking a ridiculous amount of damage. The feeling of being constantly judged by your group members isn't a pleasant sensation, but more maddening was the knowledge that certain things beyond your control -- mob stuns, lazy or trigger-happy DPS, an inattentive healer, or something as simple as a missed Mangle -- could make you look worse than you actually were.
That's why I like a recent blog post by our own Big Bear Butt on "Tips for the New Tank," which addresses not only that but also a number of pitfalls to which new tanks are prone. I particularly agree with his assertion that you should never start a boss event assuming that everyone in the group is on the same page, because someone who's not sure what they're doing can and often will cause issues for you without realizing it. It's a great guide for a new tank that gives a nod to the "feeling judged" syndrome mentioned here, and with so many players returning to their tanking trees or trying Death Knights in Wrath, I recommend it for anyone interested in the job.