(See the previous post.)
First off, believing that "most people are dumb" is an adolescent fantasy. Most people are average and want to get better. They just don't know how. What works in one area doesn't work in another area, and that dissonance causes them problems.
If you're a guild working on T4 content, take a WoW Web Stats log of your raid. Or take a look at one of the numerous WWS posts on the Raid and Dungeons forums. Over and over you will see the same mistakes. And it's not even "mistakes" like frost vs fire.
- Hunters not using Steady Shot
- Hunters missing shots
- Rogues using Shiv instead of Sinister Strike/Backstab
- Dagger rogues using Sinister Strike
- Rogues missing attacks
- Rogues using Eviscerate instead of Slice and Dice/Rupture
- Warlocks not keeping DoTs up
- Cat Druids not using Shred
- Mages missing spells
- Mages using many different spells instead of their best spells
- Shadow Priests not keeping DoTs up
You'll see the same mistakes, again and again. And at some point, you have to wonder if there's a reason that all these different people are making the same mistake, and how things could be changed to keep them from making such systemic mistakes.
Also, I'm not saying that healers and tanks are perfect. It's just that our mistakes tend to have immediate consequences, and thus we try to correct them on the very next attempt.
You do occasionally see systemic tank and healer mistakes. Tree Druids who don't stack Lifeblooms, Priests who spam Flash Heal, Prot Warriors who don't Shield Slam.
(The fact that Paladins only have 2 healing spells, one of which is spammed on a tank, makes it pretty hard to have systemic errors. About the only one I can think of is paladins not using Lay On Hands.)
Healer/tank mistakes tend to be of the immediate, tactical variety, such as healing the wrong person, or BoPing a warlock with Moroe's Garrote instead of saving it for a healer/mage. The thing about tactical mistakes is that you realize the mistake quite soon after you make it. That "feedback loop" is there, and that makes it easier to improve.