Yeah, some people are used to read shounen and when they read something they're not used to, like the fights that require skills or strategy, not tank power (which is bullsht) they hate on it, and they want the Strong to win to be unbeatable, so the manga stays stall without much progress, and some want the old generation to always rule over the new generation which is not gonna happen they like or not (proven by history, the world moves on). Or some people when the story doesn't go the way they want ( like Kaneki and Touka's relationship it builded up realistically) some don't want romance (fine but not always you get what you want), and some call it a shoujo thing (do protagonists have sex in a shoujo manga?), some want this with that and it won't happen anymore, so they hate on the manga and tell the author to die. And there's is some people when some events happen they call it "random shit I don't like it ,dropped", but the truth is they didn't pay enough attention to expect those events to happen, and enough thought. There's these kinds of people, but there's also another people who know what they're reading, who respect the author for his hard work and the fans, and argue respectably and don't say "something became sht" immediately without comprehending what happened closely (those people I would like to hear their thoughts and criticism about the manga and I'm proud to be a part of their community), anyway you can find all kinds of people in a manga or anime community.