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. 
It's not solely a reddit ism and I'm not immune from doing similar things but when it's just the same tired jokes over and over in reply to articles or whatever.
Like say it's an article about Russia doing anything and invariably there's a homophobic joke about Trump and Putin being gay together. Again and again. Over and over. Like watching a train pass by where every car looks the same.
Yeah, that shit is dreadfully common in offline conversation too.
I can handle in-jokes and even traditional cliches (Hexbear has its own for sure) but I hate when something horrid and hateful is normalized and repeated like "dae Putler and Drumpf gay amirite" or even just normalization cognitohazards like "Anne Frankly I did nazi that coming"
That quip is so old, worn out, and predictable that I audibly moan in annoyance when I see it.
Isn't it like a refurbished joke from Hogan's Heroes? "I know nazing!" Which would make the joke about 60 years old.
I do not understand how people enjoy r*ddit "humor." It's the same jokes non-stop. They will beat a dead horse years after it died. Almost every "joke" discussion turns into:
It's almost a guarantee any non-serious topic will devolve into one of the above topics and will go on for dozens of comments.
It's particularly infuriating when you're scrolling to get a source and it's just those same jokes endlessly.
And it's been this way for a decade!
Those are called unit trains and they mostly suck ass for the environment but are highly profitable for the railroads because they don't have to do a lot of switching and are typically sent to just a few customers. Examples include coal trains and oil trains. Grain trains are kind of ok, but the coal and oil must stop.