Advice and Mentoring.

Have you ever been in a group with another tank you never met before, and immediately recognize some flaws the first few seconds of seeing them? Perhaps something with their gear, gems, spec, whatever. Or see flawed techniques that made you want to speak out to them about it?
I often find myself in that situation. Not always in a random heroic either. And, with most cases when I do say something, no matter how I approach or how kindly I say what I have on my mind, more often than not its returned with a great amount of hostility.
Now granted in some cases it's because your not on your tank when you hand out these tidbits of info. For me sometimes I happen to be on my rogue when I say something, so when they fire back, I usually put it into the folder of "they think I'm a rogue and so I don't reallllly know about tanking with their class, or perhaps tanking in general.
But it also happens when I'm on my paladin. I'll get a nice wall of text or one-liner of hate. Why? Is it because you've been given advice so many times before that you're sour about it now? Or you just don't have a capacity to consider I might know what's up?
One person didn't have Touched By The Light. Turns your strength into spell power, big threat/damage talent. So I bring it up and I'm immediately shut down with "stfu, I know how to play my class, you l2p yours!" Looking at the damage meters with me on top I thought to myself "Well I've kinda mastered that one.." But why has it come to this? Way back when people at large would trade "pro tips" to each other left and right all day. But now it seems almost taboo to ask someone or talk to them.
The only real times it seems to be acceptible is if your known as quality, or you hold some sort of position of authority like an officer or class leader of some guild. When the fact is, there's tons of people in those sorts of positions that really don't know their class much at all. I remember a holy paladin in a former guild that honestly didn't know she had a HOT, or that Intellect also gaves spell critical strike rating as well as mana. I was flabberghasted. The supposed leader of my class didn't know a simple and well known perk of intellect that holds true for anyone with a mana bar.
Is the hostility towards criticism something that's just a thing that's growing and will keep growing in WOW? Where everyone thinks their elite players, or has to put that facade on because they're afraid they'll be blasted with all manner of shit talk?
Some people will appear to be listening, as in they're giving feedback that would make you assume they're taking it in, with "yeah" "oh i know what your talking about" and so on, but it's something that never shows up from them, or they never implement it.
It's come to the point in my gaming experience with WOW that anytime someone seems to be a person that will take in what you say is someone you really want to latch onto and coach the best you can. It's these not only open, but fresh minds that have the greatest chance of becoming an amazing tank, dps or healer.
If you find someone like that, keep up with them daily and see what they're doing with their character, if they've noticed anything since you spoke, or had any questions or situations they had an issue with. Or how to deal with this or that. Get that person to the level that they have a good foundation of theory, practice and awareness. After that your job diminishes in terms of tutoring time.
The questions and discussions become more engaging. The person already understands the basics, and now is fine tuning his playing. This is the time that perhaps more than any, is what will forge that persons most impacting habits.
He might favor more avoidance or go EH based on what he's seen happen to himself in combat. He might favor a certain rotation of spells or prioritize them differently because of whatever he may have done and thought back on.
This is where an important job that you play, becomes even more integral. They know what happened, and will tell you about it and their actions, at that point its your job to tell the "why" of it, and "how" you handle it, and compare your reactions to there's. Be vocal about it, talk about what you did in a similar situation and how it worked, and why it worked out for you. Lead them to the things you want to instill in them.
"Because I had done ____ not only did I keep myself alive, but it presented a situation where the healers didn't panic." "Because I saw this person creeping ahead early on, I knew ahead of time that there was a possibility of him pulling extra mobs, even after he was warned. So I made sure I had my camera in place to watch him, in case he did pull. When he DID body pull, I was already in a position to pick them up without much scrambling."
Give live examples too! In the last scenario I wrote, you could take the person to wherever, and pick a target. From there tell him/her that you'll be body pulling another another mob from a random spot.
You've already presented that something is going to be aggro'd and to be ready for it. "Put your camera at an angle that you can moniter what I'm doing, you shouldn't have to constantly readjust it to watch me, just pick an angle that's going to give you the widest view of me."
They'll be able to watch you run to wherever and pull something. The biggest part of taunting back is identifying. Once you know who and what got pulled you can negotiate an answer to it. You've taken half the work out by being ready for a known possibility ahead of time.
Planning ahead is a great tool for tanks. It allows us some foresight or anticipation based on experiences coupled with facts. The person your mentoring will immediately recognize why you panned your camera to show a wide field of view of the person. They learn it allows them to be ready for "when" it happens. Instead of sheerly reacting to the situation when it happens.
After your done with the pull stop and talk to your student about it. "You see how different it is? Think about if you hadn't had me in view, you'd of had to swing your camera around and search for me and the mob. But from watching the tendancies of the person, you knew I might pull something, and so having your camera focused on me, you were able to much more quickly pull the mob to yourself before any chance of something bad happening."

You can tell a child that the stove burner is hot, but him feeling the heat coming off the burner is what really tells him "whoa that IS hot!"

In the end you shouldn't have to hold an air of superiority. In most cases the person already knows you're an established tank, approach it like a friend. But in terms of someone really challenging you in adverse ways, don't be afraid to pull the stick out and show the person who the boss is.
This was evident in my guild where another Paladin liked to play the taunt game. Where you taunt a mob off something that's been assigned to another tank for the only reason of being that you want it and not them. If that person takes it off you, you taunt again, and this goes on until infinity..
This was little while back, when Tyrants had broke up, and I had finally made my way to Type Mismatch. We had gone to Naxx25 to continue gearing their players out. I had already been to Yogg-Saron25. I had no to even look at any of the gear in naxx, I was just there to help and make it smooth.
Myself and another tank would get our marks from the leader. "Aaesop you take this one, Darr you take that one." And, at some points in time, the paladin would just rush ahead of me and pull it before anyone moved. Or other times he'd taunt the mob right off me.
Since it was naxx, with people gearing in naxx, I wasn't concerned with my threat , I already knew I could take it easy and it'd be more than enough for the dps. But this kid, he'd taunt and start slamming spells to crank his threat.
I'd taunt it, and he'd taunt it right back off. At first it was cute. "lil guy wants some tanking action aww."
But after a while it really got old! So the next time he taunted I put BOP on him. That should have been enough to say "hey watch it." But no he clicked it off and taunted. So I pop wings, and lay into my rotation, grabbing it back without taunting.
I speak up in vent calling him out. "Look if you want to play aggro games the whole raid that's cool, you can try. But don't sit there and taunt the shit nonstop, I'll put DI on you and let the whole raid wipe, I could care less."
Sometimes you DO have to put your foot down.
In the majority of cases however this is never an issue. You'll find a comrade that's always there to have some laughs with you, discuss important matters and scrutinize new gear pieces that may have been released.
We all began somewhere with little knowledge, it's important to remember how we were then, these people are in the same spot.
Walk the path with them..