It's a large assortment of things piling up together to make one thing.

They're an image-sharing site that slaps a watermark on stuff, and rarely credits the creator, for one. KYM prides itself on giving credit where credit is due, and 9gag is renowned for claiming credit on stuff that isn't theirs.

The watermark itself pisses a lot of people off, because it claims stuff for 9gag, when they're designed to have content from other sites added to them.

On top of that, they seem to be very prideful of themselves, for no reason. The self-proclaimed '9gag army' (who stole their catchphrase from Anonymous) tries to stick its nose into all sorts of shit, such as attempting to screw with 4chan.

As mentioned, they also try to force memes, lay claim to memes they didn't create, and then overuse memes.

Any of these things alone wouldn't be too bad, but piling together in one site makes 9gag looked down upon.