9gag is an image sharing website. It's become rather infamous around the internet as the spawning grounds of idiots. Mostly due to the fact that many of the users are "new" to the internet and do everything wrong.
Using the wrong meme and the wrong time, saying "Le ____" all the time, calling various rage faces memes, pronouncing it "me-me" or "may-may".
Personally, I've found that most of the people I know that go to 9gag are the kind of people who insult nerd culture. Aka, preppy kids.
Of course, not everybody who goes there is like that, but many are.
There have been stories of 9gag "raiding" 4chan with porn and gore. Which is laughable, as 4chan deals out plenty of both by itself daily.
All in all, just a site that's generally laughed at.
I would just like to point out that, based on my own personal experience on forums, arrogant little preteen children as well as ignorant egoistic teenagers are the absolute bane of everything on the Internet.