In defense of warriors, it is perfectly possible to DPS-tank most of the lower instances, especially given the buffs to Improved Thunderclap. All the way up to Sunken Temple I used my two-hander, and I had an inexperienced healer with me. After that mobs just hit too hard. It depends on group capabilities, though; asking permission and preference is both good etiquette and good for the group.
My worst-ever tanking story takes place in Maraudon. I was running it on my 48 shaman at the time, in a DPS role; the group's tank was a 55 warrior. Level 55 is quite high for Maraudon, so it should have been a breeze. It wasn't. This guy generated ZERO threat on targets other than his primary. He wouldn't Thunderclap. He wouldn't Demo Shout. He wouldn't tab and Sunder. Sweeping Strikes? Cleave? Whirlwind? Nothing. He refused advice, too; just said "That doesn't work" and the like, and went back to what he was doing.
Being responsible, my natural impulse was to cover for him and off-tank the mobs he let escape. However, Inner Maraudon presents two unique obstacles that made this nightmarish for a shaman. First, the four-lizard pulls silence. Seeing as a lot of shaman threat is from spellcasts, this made things rough (especially since it means I also couldn't heal). Second, the worst pulls in the place are the double big rocks that, on death, spawn a bunch of little rocks. All of said rocks are immune to Nature damage, and (pre-TBC) the shaman's main threat moves were all nature based. Plus, our brilliant tank would, upon killing a big rock, target the other big rock. That means a potential EIGHT little rocks smashing our healer to jelly, and me with extremely limited means to help out.
After one particularly harrowing encounter, while everyone was drinking and recovering, our tank said something that will haunt me for a long time.
"You know what? I think I figured it out. The reason I can't hold aggro on these guys is because I'm too high-level."
No one knew what to say. There is no real response to such an astounding lack of comprehension.