At least on my server in World of Warcraft, it's becoming harder and harder to find good tanks. I was thinking about why, and I believe it comes from the basic design model of "threat."
World of Warcraft is still approximately based on the principle of tanks, healers, and damage dealers. Tanks are built to take a lot of damage without dying, but generally don't deal very much damage. Healers heal wounded players, generally focusing on the tank. Damage dealers do damage to things, obviously. The game would be too easy if enemies always automatically attacked the tank, so the game has a concept called "threat." Enemies will attack whoever is the biggest threat to it so the idea sounds good on paper, but in my mind the mechanic somewhat fails overall. It's silly that an "intelligent" enemy would not realize that he's hitting the guy in full plate armor and is not killing him and doesn't go kill something wearing cloth, but I digress. Dealing damage and healing both generate threat, so it is the tank's responsibility to generate the most threat on the enemies to protect his team. Since he doesn't deal that much damage, the class has a bunch of abilities with "increased threat" so he can not do much damage but be ahead of the damage dealers on threat.
This model drives tanks from the game for several reasons. I myself have played a tank, healer, and a damage dealer, and playing a tank is by far the most stressful job. I believe this stress causes people (like me) to eventually retire from tanking and do other jobs even if we are quite good at tanking. Most people play this game for fun and it's not fun to have to focus that hard for that long all the time to do your job well. In 5 man instances, maintaining aggro (i.e. being the highest threat) on all enemies can be nerve wracking, when damage dealers attack the wrong enemy or the healer's general healing threat pulls aggro off you. In raids, controlling multiple mobs is rarely a problem, but then you have the challenge of being the highest in threat, even compared to the highest threat generating damage dealer in your raid. The tank has some error margin, but if he lapses in threat generation for even a moment, the raid will often be wiped out because the damage dealer who is trying the hardest will take aggro off the tank. When I was tanking on raids, it pretty much felt like I was always competing against everybody else in the raid and if I wasn't number one all the time, we failed. But in a sense, that's exactly what this threat model requires.
Additionally, being the tank is the most gear intensive role because if your gear is not good enough, the group fails. If a damage dealer is under geared, he simply does less damage, the group doesn't fail. If a healer is under geared, he may not be able to keep up with the healing load, but a well geared tank compensates for it because he takes less damage. If a tank is under geared, he takes too much damage and forces a healer to heal excessively, which can generate too much healing threat and the group will fail. If his gear does not allow him to generate enough threat, the damage dealers can generate more threat than he generates and the group will fail.
Nowadays, there are fewer and fewer people willing to take on the responsibility to always be playing 100% your best and to always have the best gear possible. I now play a damage dealer most of the time and I find it so much more relaxing. If I don't concentrate too hard and I am late on nuking something, nothing bad happens. In fact, it simply makes it easier for the tank to do his job. Even being a healer is somewhat stressful, but this is mostly only in 5 man instances where you are the only healer because if you are late on a heal, sometimes, the tank dies and the group fails. However, there are more healers around in game, because raids bring many healers so as long as one or two are not late on a heal, nothing bad usually happens.
Yet another factor is that most of the really good tanks I've known have since changed their role to damage dealer. There's this sort of mini-game of who can deal the most damage that damage dealers play with each other. We all run damage meters that tally up all the damage each player deals and we can see who is doing the most damage over time. Since most of us former tanks are used to playing at such a high concentration level, we typically dominate the damage meters as well, but the difference is that now we don't have to try as hard if we don't want to. The only bad thing that happens is our ranking on the damage meter is a tiny bit lower.
There always will be a few individuals who love the responsibility of being a tank, but most people eventually give it up to relax when they play the game instead.