I leveled my main as a retribution paladin until about level 65, when I picked up two new toys running randoms -- a shield with high defense and block stats, and a one-handed sword with good stats that glowed purple and matched the shield. Yes, I became a tank because I wanted to accessorize. I'm a girl; that's how I roll.
I dual-specced protection, invested in tank gear and said to myself, "How hard is it?" I already had leveled my death knight through every heroic instance repeatedly, so I was confident that I knew the layout and the fights. My husband and I queued for a random with me as a tank and wound up in Hellfire Ramparts. And then it all went bad. I kept getting confused about who to pull first and which direction to go, and the group promptly lost confidence in me. They started playing the "kill them all quick" game, in which they used their hardest-hitting AoE spells to take the mobs out quickly and try to survive through it. Never mind that this tactic usually makes holding hate really difficult! We wiped, the healer left, the warlock and the hunter told me I was the worst tank ever, and I literally cried.
I've ran quite a few instances since then, some successful, some not. Now that I am a new level 80, I am on that perpetual quest to get lots of badges and upgrade my gear -- but I'm still suffering from tanking anxiety.