It's already been mentioned, but fucking Tumblr. The social justice morons especially. They're overtly defensive towards every minority ever and hates anyone who isn't in one. For example, if you're straight and okay with your physical sex, you're automatically a bigoted douche according to Tumblr. I don't know what about the site attracts these kinds of morons, but it does.
And don't even get me started on the fandoms. Holy shit, the fandoms. I hope you aren't following any Doctor Who/Homestuck/Sonic/Whatever fangirls. Because if you are, prepare to see nothing but slash-shipping and overreacting over anything that even remotely implies a romantic relationship between two characters (usually if they're guys).
I can't say I hate the site, but good God there's so much about the community that pisses me off. It's hard not to get mad at the idiocy that plagues it.