IMHO, it's not really younger generation thingy. Main "problem" is that many players come from single player games. Significant majority of single player games nowadays aren't really grindy and/or time-consuming, you beat it withing few hours-days, and unless it has multiple main storylines and endings, you're done with it. So people who are used to such kind of progression expect same kind of progression in MMOs.
But MMOs are always grindy (shitton of repeatable content), they have to be, cuz that's the way to retain players, esp if game is subscription based. There's also this social aspect here and there.
These things overwhelm people, they confuses gamers, and quite often cause a lot frustration.