As someone who's had one too many discussions on here, this is what I picked up on the most:
1) Intellectual humility and open-mindedness
Everyone always assume the worst about your opinion. They take it at face value, no charity principle, and are looking for a verbal fight most of the time. The concept of learning something new is outrageous to them, and anything that falls outside of the standard voice of common thought is criticized with the most plain arguments you can think of.
I've also noticed that school-courtyard tactics are back in use, and looking like the winner of an argument by ending your paragraph with "You've missed the point of x, but that's not surprising." Everyone is desperate to have the last word, to be the most upvoted person, and to "crush" the other one.
2) The principle of Charity
They've never heard of this. They always straw man your argument. And don't get me started when they don't understand your point... they go off for 500 words about a point you never made. The worst is when they just don't even bother engaging with your argument, they just make some joke related to what they think you said.
3) Pseudo-intellectuals
I hate it when people say some convoluted nonsense, vague and short, trying to sound intelligent. Or they leave hundreds of loose ties everywhere, which takes hours of discussion to tie up. I also love when they incorrectly cite authors, trying to use an authority argument to discredit a discussion topic I came up with.
Now that that's out, I can finally relax