I used to think this. It seemed inevitable, impossible, partly due to circumstance and partly due to my own choices and priorities.
My main spec is retribution. This isn't purely by choice, but is an adaptation to a guild with way too many players trying to tank. Once upon a time I did some tanking, but it's been a while. Since then I've lost my identity as a tank, since I was relegated to PUGs or randoms. Well, at some point I realized that most randoms are so trivialized by gear that it's easier to just run as ret with a mix of tank gear, just burning things down. I have no interest in PUG raids, for a variety of reasons.
The result is that my tank set is mostly stuff I picked up in ToC 25, a handful of ICC5 items, and a couple unwanted ICC25 rings. It averages somewhere around 245, meaning that it's good enough for ICC10, probably even 25, definitely enough with the buff. But that still puts me way behind the expected inflated gearscore, not just in PUGs, but in guild runs. Since we have more than enough tanks, why choose the moderately geared person? Of course this builds on itself: a non-tank doesn't get much tanking gear and the lack of gear solidifies the position as a non-tank. I could catch up a bit with ICC10 runs, but I am sick enough of it already without running a second raid with a high likelihood of PUGs.
Last night we couldn't fill the normal 25, so I joined a 10 that someone had been saving at LK. That was tricky to fill and we ended up with no priests and two warrior tanks. It didn't work well. Adds all over the place. Well, I suppose they were mostly tacked up with only a stray here and there, problem was they were stacked on me. Turns out having no short CD AoE taunt and next to no ranged abilities makes it pretty damn hard for a warrior to get aggro on adds 15 yards away (don't hold me to that arbitrary number). So they tended to congregate on the next logical spot: the person with divine storm. Sure, vigilance on that person can help, but playing taunt ping-pong just isn't much fun for anyone.
So I whispered a silly suggestion: I would be the off tank. Picking between me tanking, wiping until we quit WoW, or leaving, we went with me tanking.
It took me a minute or so to remember what I have to do to be a tank. You know, things like gear, righteous fury, switching spec, drinking again, rebuffing, righteous fury, and standing somewhere other than next to the raid poised to dart behind Arthas. And righteous fury. With that complete, I moved onto my next task: freaking the fuck out because holy fucking shit I've not tanked anything in... months? And now here I am at fucking Arthas the Lich King the last boss of this expansion unless we count the dragon which I don't, though there is something to be said for internet dragons.
Then we started it up and I immediately noticed a big problem with me tanking: I was entirely used to standing on the inside ring, meaning that if I am standing on the outside and there is a phase change, it must mean that we all have to run inside. Fortunately I was quick enough to realize that this wasn't a good idea before Arthas could cold death cold freeze me into a block of cold frozen ice cold. So not a major problem beside looking really stupid.
I'll just get it out of the way and say that Arthas didn't die and Tirion is still stuck in a block of ice.
But damn was it fun. DPS was just boring. Hit boss. Hit boss some more. Stand here. Stand here. Hit other thing. Not very engaging.
In contrast while tanking I was actually watching things, needing to think a bit. As DPS I tend to go into a daze, which works well enough because I react by habit to bosses moving, even without dialog, as if it were a dance that I've done way too many times. This doesn't work perfectly when things change, as when I was tanking and therefore standing somewhere else, but that's an unusual situation.
I had to figure out how to grab three adds which spawn around Arthas and therefore in convenient range to get hit by the tank and the fury warrior. Ideally all three would glance at the fury warrior, at which point I can AoE taunt them off him with no mess. Less ideally, one gets on the tank, in which cause I use my single target taunt. If two get on him, it's a bit more trouble, since AoE taunt will bring Arthas, but I also need my taunt for the one still on the fury, and meanwhile I have stuff already on me, blocking my view somewhat. So I throw a shield and preemptively taunt for the damage while they're crawling up and constantly try to hit my macro to fire my rocket gloves which doesn't work because my tanking gloves have a couple inches or armor plating on them rather than a rocket launcher like my DPS gloves.
It's a more interactive process.
In the end I was able to control the adds very well, though I still had to be careful with target of target taunting. And my rotation was completely fucked. But I figure that was a lower priority than keeping adds from eating the other tank, the DPS, or the other DPS. Or the healers.
So you know what? I was wrong: I can tank ICC. Or at least one boss for a couple phases, and I plan to do it again next time we raid. Dead boss or not, it's fun, and that's what matters.