What bothers me most is very long posts with no paragraphs. Though to be strictly accurate, they don’t bother me, since I CBA to read them.
There should always IMO be a Pedants’ Corner, for people who really cringe at basic mistakes - I do - to let off steam.
Having said that, I only ever corrected someone once, on another forum, after that person had corrected someone else - and made a basic mistake in their own post! Pots and kettles....
However I often think that people who do make basic mistakes - it’s/its, your/you’re, would of, loose/lose, etc., maybe just don’t care, because they’ve been led to believe that these things really don’t matter any more.
Which is sad, because they certainly do, and e.g. basic mistakes on CVs can really affect someone’s chances of a job.
Worst case of grammar-superiority I ever saw (on a very different forum) was when someone used a Latin phrase incorrectly, and someone else pointed it out and corrected her - on the open forum, instead of by a PM. It really made me cringe for the other person.
I knew the ‘corrector’ - it was entirely in character. She thought herself superior in so many ways and was often keen to demonstrate it. It was ages ago but I still really regret not PM-ing her to say that was an appalling thing to do, and moreover Very Bad Manners - that might have gone home since she fancied herself so very correct in everything.