There's a difference between learning what does what and learning how to use every skill you have effectively. A lot of players fail at the first step, only learning what 5-6 skills they'll use the most and ignoring the rest. The result is Paladins who don't heal others, Warriors who tank in Berzerker Stance with a 2H, and Druids who aren't aware they're different from Rogues.
I also find it quite strange that people continuosly bash WoW for being so incredibly easy, and how everybody can learn to play it in a matter of minutes.
How can it be then, that a very large percentage of random groups you do instances with, fail miserably because someone don't know how to play?
Even if the instance run is successful, it could always have been done faster and more smooth.
I think many people believe they play perfectly, even though that's not the case. And unfortunately most people are reluctant to point out these people's weaknesses - the most contructive criticism that usually comes accross is in the style of "OMG L2P noob, WTFBBQ!!? LOL LOL you suck!!1". Which of course not is taken seriously by anyone.
And no, it's not just about having good gear. A player with good gear can still go total pew pew on some mobs,
outaggroing the tank by miles, and maybe think he's the best player in the world because he's on the top of the damage meter. Etc, etc.