1. Watching mitigation cooldowns
Tanks have cooldowns that mitigate damage. In the days of yore, these cooldowns used to be affectionately referred to as "oh, shit" buttons. You'd be tanking along, laughing at bosses, and suddenly your healer would lag behind or something. "Oh, shit," you would think to yourself. That's the moment when you'd hit the button. It was a stopgap measure to help your healer out.
The modern world of mitigation cooldowns doesn't work that way. Nowadays, if you don't press the button, there's no point to the button. You either use your cooldowns strategically and constantly, or you save them up to handle boss mechanics.
Let's compare Shannox and Beth'tilac for this purpose. When you're fighting Shannox, you tend to use your cooldowns in a smooth, even fashion. You're using those cooldowns constantly, saving the healers mana over time. You also make sure that you have them available at the end of the fight, when Shannox hits harder -- but overall, cooldowns get used consistently.
On Beth'tilac, a tank has to go upstairs without a healer for a few seconds. While in the upper web, the tank is getting hit hard with few heals. So in that fight, you tend to save your cooldowns for those first few seconds of each phase.
I would argue that having systemic, total control of your cooldowns and knowing when to use them is a much bigger part of tanking than maintaining threat.
2. Providing interrupts
Tanks haven't always had interrupts, but now every tank does. When tanks received interrupts, it's like half the DPSers in the game suddenly forgot how to use their own. Through this mixture of player zeitgeist and design, tanks should now be hot and ready to slam an interrupt in a moment's notice.
3. Not standing in stuff
Okay, so everyone in the raid needs to not stand in stuff. That's hardly a huge statement of deep epiphany. However, avoiding fire becomes a lot harder when all you can see is the boss's crotch. It also gets tougher when you have your finger poised over a cooldown, waiting for the exact second to mash an ability that will save your life.
4. Not forcing you to stand in stuff
Really, though, I mention not standing in stuff as a precursor to talking about not forcing the DPS to stand in stuff. Boss positioning can be tricky at time, and the tank has to move the boss in such a way that DPSers can do their jobs without forcing them to stand in fire.
Beth'tilac is another good example of this. When you go upstairs to the web, you need to keep an eye out for meteors and huge spots of flame. If you fail to keep an eye out, you'll fall from the web to your doom.
The tank could just find his own place on the web and leave the DPS to sort themselves out. But that's a bad move; fire builds up, and it becomes far too easy for DPS to accidentally fall through. A kinder, better tank moves Beth'tilac from time to time, giving those DPS folks enough room to get their job done.
5. Preventing cone and cleave attacks
I feel that there are many fewer cone and cleave attacks in the game now than before. That being said, there are still plenty of mobs and bosses that unleash a frontal assault. In this case, the kind tank takes the time to face the NPC away from the group.
6. Not outranging the healers
While healers will eventually have to move during many fights, there's still an important factor that healers must deal with. Healers have cast times; tanks don't. The kind tank will keep an eye on where his healers are hanging out and will put forth the effort not to get out of their range.
7. Watching the healer's resources
The tank executes the pull. That's the way it works. When you're ready to start fighting mobs, the tank is the one who starts the attack. Therefore, it equally falls on the tank to keep an eye on healer's mana, resources, and buffs.
If the healer isn't ready for a pull, the tank has no rightful business pulling. You don't get to just charge ahead because you're the tank. You set the pace, but if your healer isn't ready, you will simply die.
8. Grabbing adds quickly
I argued with myself about adding this task to the list of things "you don't know," but I realized that to a newish player who hasn't tanked, it might not be obvious that the tank needs to grab those additional mobs that show up. More importantly, tanks need to grab those adds quickly.
As a general rule, I give myself the space of a GCD allowance for doing a good job. If I get the adds in one global cooldown, I did good. If I grab them in the space of two GCDs, I did okay. Spending three or more GCDs grabbing adds means I failed in some way.
In the end
Of course, this list could go on forever. I'll probably do similar lists for healers and DPS later, but I wanted to talk about things tanks should be doing besides just maintaining a threat rotation. Aggro is obviously important in fights, but that just doesn't tend to be where my mind is dwelling during most fights.
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