I'd have to say that the two most fun things that are actually part of tanking are:
Being one of the last people standing after things have gone wrong, and just holding on and holding on and holding on until it's over. On the one hand, that kind of thing only really happens when things go wrong. On the other hand... it really gives the feeling of "Oh god... time to rest". I have this mental image of planting my shield in the ground, planting my ass on it, and breaking out a canteen to cool off while the healers go scrape everybody else up off the floor.
That's a GOOD feeling.
The other really good feeling is saving people from things, including themselves. Intervene was an awesome new ability in BC, because it gave us a way to really quickly get to the person who pulled aggro and *do* something about it. It's most satisfying when you've just had the most squishy person in your five-man pull aggro on something they really shouldn't have (big melee type), and you intervene to them and *taunt on the way past*. Hah! Take that, you brute!
In general, I think the important part is: anything that makes you feel like a hero. A good tank takes pride in holding aggro and taking little damage while being beat down. But it's not heroic. It's the bad times, and the ability to survive them, that makes tanking the best. When the healer goes down and you just barely manage to both hold aggro and avoid dying long enough for the party DPS to finish everything off. When all of the DPS goes down, and you and the healer bully through the grueling effort to finish the enemy off. Standing at a flag in Arathi Basin with five guys beating on you for thirty seconds asking themselves "Why won't this guy DIE!?!"
These disasters are the golden moments that invoke a visceral response. Technical tanking (maxing TPS, avoiding bad stuff, etc.) is enjoyable... technically. But that adrenaline rush and knowing that people are depending on you to survive. That's something else.
Learn to science and stop theorycrapping in its tracks.