Okay I have 2 I would like to share, one very minor but the other bigger.
Minor one is I do not like when people try to assign positions to groups that
already have positions (it's a little different with no position groups because there's no right or wrong positions in these cases so it's more subjective). Of course, people are allowed to prefer one dancer over another, or one vocalist over another, but discrediting those with main/lead positions thinking you just know better than the members of the group and the compant that trained them is kinda ridiculous. The companies don't give out positions willy-nilly, they have to think carefully about who's getting what based not only on talent but how well they can manage this position. THE WORST IS WHEN PEOPLE DO IT WITH LEADERS. How can you sit there and think to yourself you know who would be better suited to lead a group than the group themselves who trained together all this time. You don't know what's going on behind the scenes so how can you act like you know who'd be better at managing these people.
As for the second, I touched on it briefly in the comments of
@sanasideup status, however I want to elaborate more here: I think the main reason people hate minors debuting is because they're jealous. Of course, the K-Pop industry is very cut-throat and ruthless so I do think a lot of people have genuine concern about under 18s entering the industry, but especially with other under 18s criticising the choices of these idols parents and the idols themselves, acting like they should be living a "normal life like everyone else", yeah ofc it's sad to think you "don't get a normal childhood" but are you sure there's not this feeling of "how come I'm sat in school struggling to make friends and feeling crappy about the way I look" and then seeing someone your age becoming internationally famous, with thousands of fans spending money on them, and they get to wear posh outfits and wear nice makeup and perform in fancy arenas. Like yeah it's normal but it's a hell of a lot more interesting than a normal life, and I think that might be a driving factor to why people hate it so much, even if these are subconcious thoughts