Just when I thought this phenomenon was gone, I up and see not just low level newbie tanks doing it - but a self-proclaimed RL/GM teaching his tanks to do it as well. What I'm referring to here is the ever unpopular, wildly unexplained, Taunt Pull. That's right, pulling a boss/mob by using your Taunt first.
Now don't get me wrong. I can see where this habit starts, and why it probably takes far too long to correct. Unfortunately, it's problematic in later dungeons and raids - and I'll explain why in a moment.
How it all starts
For the Paladin, this starts when Righteous Defense is granted at level 15 or so. This ability acts as a ranged pull, does a significant amount of damage at that level, and is one of the few 'moves' in the leveling Paladin's arsenal. Since they don't get Avenger's Shield for quite some time, it becomes ingrained to pull this way.
For the Warrior, this comes from their gun/bow being a stat stick - and the general lack of ammo in their bags. Because of this, they either taunt pull, or (thanks to talents) charge into the fray - generally leaving the healer behind.
DKs don't get a taunt for many levels, they are usually the lowest offenders of this. The biggest issue I see with DK tanks outside of raids is not using Death and Decay because it "costs too many runes".
Druids get Faiere Fire at a low level, and learn to pull with that - leaving their taunt alone. Some druids still pull with taunt though, however they are on the lower end of the offender chain.
Do you see my problem here? Not only is it people pulling with taunt, but it's primarily Warriors and Paladins doing it! For shame.
The real issue
Pulling with your taunt is, well - I have an analogy Reaganomics used in the past that isn't really appropriate. The clean version is - it's idiotic to pull with your taunt (even on un-targetable bosses). Here's why.
Taunt has ZERO positive effect if the target is already targeting you. Only the Paladin taunt does significant damage I believe (if they aren't targeting them), and what you've now done is trigger your taunt CD and started the taunt immunity timer on the mob/boss. Let's look at two different examples.
Example One: Taunt fires first. So let's say you're using a Warrior to MT Saurfang, and because he knows your DPS is sitting there spamming the /AMGDPS key, he's frantically spamming his taunt - just waiting for Saurfang to go active. Suddenly he's live, and his taunt fires - along with his auto-attacks. Now you've got three seconds where the boss is focused on your Tank, but he's got 1.5 seconds waiting for his GCD to come down. You have time for one, maybe two abilities that aren't on the GCD to fire before the effect wears off.
Now in this instance, JoeJoe the DPS pulls ahead of Tank 1, so Saurfang targets him. Your MT is standing there holding his junk in his hands, because his taunt is on CD, so tank #2 taunts. You now have two taunts in < 8 seconds. A few seconds later, the MT taunts back, because he's supposed to have Saurfang first. Oops. Now the Boss is taunt immune - right around the time that Rune of Blood comes out. Suddenly Saurfang is healing up that damage that your overeager DPS did, and his Blood Power is rocketing up because he still has 10 seconds of Taunt immunity left - and your OT blew their taunt anyway because he didn't realize Saurfang was immune.
Example Two: Tank fires taunt after establishing agro.
Precisely zero happens to his threat, it does NOT force the boss to attack for any length of time, and just starts the taunt diminishing return/immunity timer.
Example Three (Bonus): Tank pulls trash with a taunt.
Overeager DPS guy opens up, trash turns and punches them in the face for TEXA$ worth of damage - because the tank's taunt is on CD.
Raid Leaders and GMs, please stop teaching your tanks bad mechanics. Instead teach your DPS to be patient for two or three seconds. The days of waiting for five sunders is past, but if you don't have a hunter/rogue doing a threat transfer, the tank still needs a few seconds to establish solid agro. Your healers (and DPS) will thank you for it. Trust me.