It's a lot of work to get into end game and begin raiding in vanilla, the boss mechanics aren't really what make it difficult but the large amount of players required makes the odds of fucking up pretty high. Vanilla is an entirely different game, it has its own walls to climb over and since gear isn't thrown at you like on retail it's gonna be interesting to see the statistics. The majority definitely will not be raiding I can say that for sure. Even the leveling dungeons are going to kick a lot of players asses because you have to CC and pull very few mobs or even 1 at a time or it's a wipe.
I mean we can just look at cata heroics and see that even small amount of difficulty and CC requirements destroy most PuGs. So while we may be more 'skilled' it's a different ball game. If you are pro at basketball it don't make you pro at football even though they're both sports.