They despise anything that deviates from their interests, frequently attempting to make you stay away from:
Genres that they possess no interest in.
Political commentary that they disagree with.
Pretty much any attempt to actually have high-energy fun. No memes, shitposts, etc. If you're extremely lucky, they'll consider conventional forms of artwork to be acceptable.
If you frequently post about any of the above, they're going to go out of their way to make sure that the moderators censor them. Even if that doesn't happen, they're still going to call you retarded, a freak, evil, et cetera.
Post-wise, they usually talk about:
When debating politics, they're going to attempt to create circlejerks. If they see someone they disagree with, they're very likely to avoid any sort of argument more advanced than ad hominem.
"Popular" media, or at least media that is popular inside their circles.
I've also noticed that a lot of these people are either aggressively left-wing (but not politically learned enough to understand that Bernie is just a tax-loving capitalist) or live in a stereotypically left-wing area.
All of this usually leads to a poster who is uninteresting to talk to and often detracts from interesting debate. Even the mentally ill are better than these people.