The four tenets

In my not so humble opinion, there are 4 visible aspects of tanking that can be resonably gauged. These are (in no order of importance, a.k.a. equally important):

1. Reaction

2. Threat

3. Survivability

4. Adaptability/Judgement

Threat is pretty self-explanatory; how much you put out directly affects how much your DPS can unload. Any good warrior can put out a ton of it.

Survivability is directly related to your gear and how consistent you are at keeping your debuff uptime at 100% on a mob. Assuming the debuffs are always there, your level of gear and your positioning is all there is to surviving.

Reaction is an important one. This is how fast you Taunt a mob back when someone pulls it off you. This is also how adept you are at refreshing your debuffs on a mob, whether your shouts are up, etc etc.

Adaptability is an interesting one. The way I see it, apart from 1, 2, and 3, the only remaining practical aspect of tanking involves how good you are at judging a given situation. Do I need to press a cooldown? Which cooldown do I press? What will happen if I move the boss here?

Am I about to lose aggro? What do I do if that DPS is about to pull the mob off me? What skill do I press to prevent that?

This one takes time and experience to hone, and it's something you need to learn mostly by yourself. A split-second decision is often all the difference between a kill and a wipe. If you play a warrior and you're tanking, YOU will be walking that fine line between madness and sanity all too often. And the lives of your party or raid members depend on it.

As for me... hmm. I'd rate myself a 6.5 on reaction, a 9 on threat, an 8 on survivability, and a 7 on judgement. How do you score? How do I score in YOUR opinion? I'd love to hear it.

So there we go... practice, practice, and more practice... and you'll get there someday.