Reputations linger, and the impact they have on tank choice is real. Tanks will be swapped based on something as small as a belief that x class will make a small difference in how much healing will be needed in order to learn a hard mode fight.
We want to see more paladin and druid MTs, and part of that includes raiders accepting them as legitimate tanking classes, and those players feeling that if they go through the effort to gear up that we're not going to banish them down to OT status again. Similarly, we wanted to slow down the number of groups that appeared to be swapping to a DK for their MT, seemingly permanently, because they were convinced they would just always be the best tank. We have no problem with warriors being the most popular tank (and they still are) because there is a long legacy there and a lot of those characters have been around awhile. But we also want the other 3 tanking classes to feel legit.
Why are warriors still the most popular tank, then? It's possible to argue for legacy again here. Warriors have been an accepted tanking class for the entire existence of the game, to the point where if you say 'tank' sometimes you still have players who automatically think warrior. This blade cuts both ways, of course; being a DPS warrior you'll hear a lot of people telling you that you should be a tank instead. That's what the player base expects from warriors. Does this mean that they want to see less warriors as tanks? GC says "Nope. It's not a population issue. I hope that's not what people take from my post. There are more warrior tanks and there always have been. We see no reason to try and nerf our way to 25% of all tanks being warriors."
Now, I've seen arguments back and forth that this means that warriors aren't going to get tanking buffs they need purely because there are so many of them. To be honest, as a warrior, I kind of don't see any major buffs needed. Other tanks might need rebalancing down in some areas and up in others, but warriors feel like a very solid, very flexible tanking class to me, albeit one that requires a significant investment in time to figure out fully.
In comparison, tanking on my DK feels ludicrously simple compared to my warrior. The warrior is clearly very fiddly in comparison, even after I built a macro that auto-spams Heroic Strike for me so that I don't have to endure any more carpal tunnel HS pounding. (It feels like I'm cheating to use it, but until HS is a more interesting ability for a tank than 'spam the heck out of this' I'm going to keep my macro.) The warriors array of cooldowns and tricks compares favorably to the DK: in general, tanking on the warrior actually feels like tanking, while tanking on the DK is very close to how I DPS on him and has less of a 'must build threat now' feel to it. I'd like to see warrior boosts to AoE threat without making Shockwave spammable (if anything, warriors as tanks have been moving away from ability spam, I don't want us to lose that).
The problem between tanking classes, in my opinion, is directly locked into the increase move towards difficulty in raid encounters being based solely on instant gib moves from bosses. Telling tanking classes to stack more threat abilities is all fine and good as long as you're also not asking them to go in and tank a boss that can rip their heads off in one or two hits. And any tanking class inequities that are perceived by the player base will be dramatically emphasized by a boss that can more easily instakill class X than class y.
It is impossible to balance all four tanking classes so that they all have comparable abilities for any and all situations in the game. Making them all viable 5 man tanks was in of itself an amazing technical challenge. What we need is a series of encounters per raid that emphasize all four classes strengths so that you'll never want to sideline anyone, while still allowing other clases to be capable of tanking said encounters so that raids will never again be in the Mount Hyjal/ZA Dragonhawk situation of "bring X tank or go home". In other words, it's not the classes that are the issue in current tank design, it's the encounters built around massive spikes and gimmicks that can ruin an entire attempt in 2 seconds, and these encounters will always create perceptions of tank superiority in the minds of those players working on them.
I'm not arguing that there are not issues with each tanking class that should be addressed, nor am I arguing that warriors are fine and should L2play, a phrase I find pernicious and irritating. I'm arguing that the nature of tanking as it stands at present divides tanking into two (morphing into three as 10 man raiding takes on aspects from both 5 and 25 and hybridizes itself) almost completely different games. In the first game, pretty much any tank will do. In the second, decisions are made on razor-keen distinctions, and as long as that game is predicated on cooldown rotations being king, some tank classes will always feel weaker than others and real concerns will always be sidelined by that distinction.
Until this situation is addressed, the inertial pressure of so many warriors tanking will work against the class even as other classes are touted as the tank which makes most encounters easier. Warrior tanks will always be the standard by which other tanks are judged and judge themselves, purely due to their ingrained popularity. This is the sword that hangs over the class' head.