I hope this isn't too contentious but I'll try to cover most things. I've posted this a few times, but I checked out 4Chan about twice in the early days and saw CSAM both times and it gave me personally a visceral hatred of the site. I've heard it got better/that's not representative but it's a hard thing to shake. The origin of the site is also supposedly Moot getting kicked off SomethingAwful for posting 'lolicon' (child anime porn). They've also gone after and doxxed pedophiles though, so the sites relationship with that sort of content is... complicated. I think most of the worst ended up moving to 4Chan clones quite awhile ago because it really splintered again at some point and became known as the cleaner Chan board.

It's also known for its extremely abrasive mildy sociopathic culture and 4Chan posters have a very samey 'posting voice' where if you don't like it you can hate it. It permeates a lot of the internet, but 4chan is kind of seen as the epicenter. I think it also gets blamed for a lot of negative internet culture like doxxing and choosing targets to harass, although I'm not sure how much of that was actually 4Chan. I think most of those people moved on to Kiwifarms. 4Chan probably gets some hate for things that other Chan sites have like Qanon in a sort of 'you started this' way.

And finally the politics are complicated. It actually used to be slightly left leaning or at least libertarian or anarchist, but over the years pol in particular has been known to be hard right wing. It definitely seems like they had a shift in political tone for the (IMO) worst at some point.

Personally I won't hide that I'm a hater and an unapologetic curmudgeonly old man, but that's my perception. On the other hand if you think the CP stuff is overblown, don't care about the negatives because there are apparently good boards there that are insulated, or are just hard right yourself then it is one of the last major discussion boards on the net. Some of that's probably out of date (like I said I gave up on it pretty quickly) but I'd wager most people with negative opinions are thinking of one or more of those. I'd be interested if any haters have other reasons.