About half a year ago I did an experiment with my priest, tried a shadow spec, researched what the current builds were, asked my readers for help on the spell rotation, and practiced that rotation on a dummy. In spite of wearing mostly "healing gear", I managed to get a spot as dps on a Naxxramas raid, and ended up doing 3K DPS during that raid. Which means that under the definition of a certain goblin, I'm not a "Moron & Slacker". In fact you could even say that my performance was in line with said goblin's theory that gear doesn't matter as much as skill. On the other hand, while "holy priest gear" isn't totally perfect as "shadow priest gear", it is nevertheless close, so the fact that I was wearing full epics certainly helped.
It is certainly true that some players are just plain bad at playing their dps class. I recently was in a group with my protection warrior as tank doing a 5-man dungeon, and at the end of the dungeon I looked at Recount (a damage meter addon) and was surprised at seeing that I had dealt more damage than the dps death knight in the group, who was at just over 1k dps. That death knight could certainly improve his overall performance by a significant margin by looking up a good talent build and spell rotation, and practicing it a bit on a dummy.
But dps in this discussion is the simple case. There are usually 3 dps in a 5-man group, all having the same role. And that role is easily quantified with any damage meter addon. Even taking into account variations of gear, it is easy to see that the guy who deals 3K DPS is a good damage dealer, and the guy who deals 1K is not, especially if the two just went through the same encounter. And for dps the spell rotation that maximizes damage is usually independant of the target. That is you can look up a spell rotation for your class and talent tree on the internet, practice it on a dummy until you master it, and you'll do well enough in most dungeons and raid encounters. I'm certainly not calling everyone who doesn't do that a moron, because it might simply be a case of not being interested enough in the kind of encounters that require higher dps. But if you *want* to improve your dps, it certainly isn't rocket science to arrive at some decent output.
For tanks and healers the matter isn't quite as simple. That starts with the problem that you can't use the training dummies to practice your craft. Yeah, you can first damage the dummy and then heal it to measure your heals per second with various healing spell rotations. But as the famous How To Top The Healing Meter jokes show, being the best healer and having the top heals per second on some meter are two very different things. I would be very, very careful before making a screen shot of the healing meter at the end of a 25-man raid, and then pointing fingers at those who aren't as high as the others. Coming low on a healing meter can be a function of having less fast spells, or it could simply be a result of the healing assignments. And unlike dps it depends a lot more on the nature of the encounter. If you reset the healing meter before each boss fight, you'll have encounters in which one class tops the healing meter in one case, and another class in another case, depending on whether just the tank, or just a few players, or the whole raid gets regularly damaged in the encounter. And how do you practice tanking on a dummy, or measure it with a damage meter?
The other big problem of measuring the skill of tanks and healers is that in a 5-man group there is normally only one of each. You can compare the performance of the 3 dps, but you don't have any comparison for the single tank and the single healer. The only thing you *do* know is whether you succeeded in beating the dungeon or not. If you did, both tank and healer were obviously competent enough. If you didn't, in spite of a tendency to always blame the healer, it is totally possible that either the healer was bad, or the tank was bad, or even that the dps messed up aggro management. For example a mage who does AoE damage in a situation where he shouldn't have will look perfectly good on a damage meter, but still be responsible for the wipe that followed.
The underlying reason that you can't practice tanking and healing on a dummy, or measure it very well, is that both activities are a lot more interactive than damage dealing. Which is why I play them in the first place. But the theory on tanking rotations and healing rotations doesn't reach further than simple tank'n'spank fights against a single boss, and there are barely any left of those. Being a good tank requires you to be aware of every mob in the fight, and putting the right priority on managing the aggro of each of them. Being a good healer requires you to decide who to heal, and with what spell, not just producing the maximum amount of healing per second.
And to close the circle and to come back to the eternal gear vs. skill debate, it is certainly true that a tank is more gear dependant than any other class. Tanking gear is more difficult to acquire than any other sort of gear, for example it is impossible to get tanking gear as reward from PvP, and it is underrepresented on every loot table. It is much, much easier to get a set of gear together that allows you to reach 3K dps than it is to get gear together in which you have 540 defense, 30k armor, and 30k health unbuffed. (If anyone knows of a guide how to get to that sort of warrior gear the fastest, I'd be very interested.) PuGs don't even invite tanks for lack of gear. Any guild knows that if their main tank walks out, he is far more difficult to replace than their best healer or their best dps. I have a faint suspicion that Blizzard is regulating the speed of guild raid advancement by limiting how much tanking gear they hand out.
So in the end we do have a system evaluate the skill of dps classes, even if that measures the mix of skill and gear. But for healers and tanks we don't have any good way to measure skill at all. And so we are stuck with "blame the healer" when a PuG wipes, and tanks being judged solely on their gear. That not only negatively affects the eternal lack of tanks and healers, it also makes it a lot harder for those tanks and healers to improve their game.