Redditism 1: "It's the internet. You are allowed to swear." 
I hate when some very grown-ass adult says that because someone didn't cuss enough for their taste. I swear all the time in my posts here but I still find that shit really, really tryhard and it seems more immature than not saying the naughty word to me.
Redditism 2: Ending a rebuke with a question mark when it's not a question to make it sound extra snippy.
I've heard this being compared to a "vocal fry" and maybe it is, and coming from CA, vocal fries were often said out loud as a form of subtle hostility toward people perceived as lessers, such as retail and restaurant workers. If you need an example of what I'm talking about, it usually goes something like this:
Poster: "I think (opinion)." 
Redditism enjoyer: "You're wrong?" 
Redditism 3: "Do you need help? Who hurt you? Help is available if you need it, buddy!" 
This one is the worst one I can think of right now because it contaminates even the very possibility of showing sincere care and concern for someone else. It comes loaded with the implication that the person that was "hurt" or "needs help" is fundamentally wrong and should shut up. Fuck that ableist shit, forever. 
Their humor is awful.
Every sub is astroturfed or a government psyop, but they will pretend like their sub is not. Any fandom, hobby, or whatever niche will always have corporate stooges posting on behalf of the company. Any criticism of anything in those topics will be attacked with vitriol and real users will join the side of the stooges. Hall monitor getting buddy buddy with the campus cops energy.
Their taste in art is awful. R•dditors will only go bazinga for paintings of attractive women, pop culture shit, and "realistic" art. Any abstract art or art with historical significance is frowned upon. Any experiments people do in niche communities are given contempt. The truth is, their taste is completely kitsch. Your aunt with Bettie Boop posters and Minions furniture has more artistic sense.
Their taste in music is awful.
They are deeply unserious about film. Yes, Requiem for a Dream is disturbing. No, it is not the most disturbing thing ever filmed. I'm not trying to be an edgelord. I just think Come and See, Antichrist, or Henry: Portrait of a Serial Killer are all more messed up than two women sharing a double dildo. Also Quinton Tarantino is a hack.
Referring to r•ddit outside of r•ddit. YouTube comments do not link to anything when you write "r/whoosh." There was nothing to be gained by adding "r/" other than making yourself look like a dipshit. Nobody on Instagram understands your stupid fucking bacon whale joke or whatever the fuck.
They are flat-out wrong about so much shit I cannot believe others believe what is being said. For example, they will make predictions on Pokémon cards for people to "invest" in and end up getting dunked on by Nintendo or whoever runs Pokémon right now. But nobody learns the site is full of shit and they will continue to lose money. Hint: collectibles have never been good investments.
That's all I have for now. I do not go to that site. I've never had an account there. Occasionally in the past I'd stumble onto something from searching, but that was enough exposure to make me hate the site. Unfortunately, they have spread to other parts of the internet and exposed us all to brainworms.
A Serbian film was the most disturbing film for me. Not in a way that had me thinking for days like come and see and the other films you listed but in a "did this motherfucker really put that shit in there?"
I've never seen it and don't want to. I know about the ending and whatnot, though. I think even putting it to film was more messed up than RFAD. A group of people wrote it up, showed it to other people, and they all decided "Yup! This is what we're gonna spend thousands of dollars to film!"
Honestly the worst part about Requiem was the stuff with the older lady on dieting pills. You have to worry about elderly people isolated from the outside world plummeting into psychosis.
Thats a good point. The director making it from her POV while confused and scared was well done.
How else are we gonna warehouse them?
People here legit had a struggle session over fake shit from that relationship advice subreddit.
A lot of people who post on reddit are kids. I was posting on reddit when I was 14 back in 2015.