When I first played World of Warcraft, I had no idea what I was doing. However Blizzard did a good job making sure new players were taught. They did this by using help tips throughout the game, and also by easing you through the skills as you level. For example: As a paladin, you pretty much used holy light as your main healing spell, but as you progressed the healing spells become complex. In order to use them properly, a proficiency is required in healing. To use them in a raid required the healer to know his spells by the time he was level 50 or so. So everyone at a certain level knew what they were doing. Blizzard did this through setting up the environment to challenge their abilities.
From what I saw, most people did what they were suppose to do, unless they were complete ninjas. (Someone who steals from the party)