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. 
I’m with you on this one. It’s intended as a sort of, “we all know this but don’t say it,” or similar, but always just comes off as condescending to me. Maybe because it’s so often applied to statements that are in fact known and acknowledged.
“Unpopular opinon”
I just remembered another because of your post: “Let’s face it/let’s be honest” used as magic words to transmute opinion into ironclad objective fact.
I think there’s some value in an “I’ll be honest,” as a signal that one is being forthright despite a potential cost, with a corresponding appeal to the listener to hear the speaker out. But yeah, when it’s an appeal to an imagined silent majority that shit is often not very good.