So with the release of Cataclysm many people have stated that skill has been put back in Warcraft. To me I am going to say that it was never taken out in Wrath. The issue in Wrath was heroics, bosses, and such were more forgiving for mistakes people made. This never took out the skill in the game just made some players appear better then they actually were. Let me explain what skill in WoW actually is.
Skill in WoW is as simple as knowing. Know what you are supposed to kill, know what you are supposed to heal, know what you are supposed to tank, know where to stand, know where you can't stand, know when to move, know when to interrupt, and so on and so forth. In WoW knowledge is your key to being successful. The second component to being skillful at WoW is how to adapt. Yes you may know that you can't stand in fire, but if you don't move out of it then the knowledge is nothing. You have to be able to react to situations around you. So skill in WoW comes down to your knowledge and ability to be in able use that knowledge to react.
So this didn't change the people who always do this better will be the ones who will be those players that everyone sees as skillful. Just like anything you can become a better if you practice these things and you will become a better all around player.
So really to be a skilled player it comes down to don't stand in crap that hurts you and kill the thing that is killing you. You understand that phrase and what it means and you will be fine.