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. 
Citing any of these
https://en.wikipedia.org/wiki/List_of_fallacies#Formal_fallacies
The ones that really make my eye really twitch are:
It's not a formal debate and ad hominem isn't some fancy Latin term for an insult. Imagine if someone's response to being insulted was "You just insulted me!"; yes, you got insulted. Congrats on figuring that out all by yourself.
This one comes up so damn often when a person establishes a definition. "You cannot be a pacifist and support the death penalty" kinda thing. It comes up most often in discussions about socialism/communism from the mouths of reactionaries. The NTS fallacy is inherently tautological by its definition; it must be something where the definition is used to dismiss evidence to the contrary. "No human being is taller than 3m" is not committing the NTS fallacy. If someone said "No human being is taller than 2.5m" and someone responded "But Robert Wadlow was over 2.7m" and the reply was "Well that means he wasn't human" that is the NTS fallacy. Otherwise every definition ever, and in fact every word ever, would be guilty of the fallacy as every single one of those necessarily excludes something.
This one comes up less often but it's just as obnoxious. You aren't having a formal debate and there is no explicit agreement about the bounds of the discussion. Anyone can bring anything up. If you don't understand why something is irrelevant then that's on you, you can ask. If something is completely irrelevant then you can just say "That's irrelevant" or you can ignore it.
Remember that one kid who would kick a ball around with your group and they'd always try and rules lawyer someone being out of bounds when everyone else is like "Bruh. We're just kicking a ball around for fun. There is no field, there are no painted lines, there is no umpire. Chill out."? This has the same energy.
God I remember when I was 14 or something, this wasn't on Reddit but I inexplicably became fascinated by pointing out fallacies. It did not take long before people told me I was being incredibly annoying, clearly not understanding what I was talking about, not contributing anything meaningful to any discussion, and just generally being a smartass making a fool of myself.