Difficulty wasn't the problem in Wotlk, the problem was that even hardcore raiders were incentized to do LFD dungeons, who obviously wanted to speed through these dungeons as fast as possible.
Once ended up on my fresh 80 tank alongside a 25man geared Resto Shaman who basically wanted to do the whole dungeon with ever stopping.
Not that i did mind that, but someone who was just getting familiar with the endgame and the tanking role would have seriously struggled in that situation.
That's just the problem, you can't put a veteran who wants to complete a piece of content as fast as possible next to a new player and expect it will work without any issues.
Or not even a new player, just someone who is looking for a "chill run".