I hate terrible grammar. It annoys me if I have a typo in my posts and if I spot one, I must change it. Well, online I can understand some poor grammar such as odd capitalisation (for EMPHASIS) and multiple pieces punctuation, etc, because you can't sense tone or see facial expressions...
It also annoys me when people protest against me correcting their grammar or pointing out that if they typed a little better, they might just pass off as a cat jumping on the keyboard. If you're speaking to me in a manner such as "lol lyk wot r u doign my grama is fyn lyk lmao y do u evn cair how i typ anywy lol"
If it's little grammar things such as saying "who" instead "whom" or "me and X" instead of "X and I" then I can understand them not wanting me to ALWAYS correct it, sometimes I slip up on those, but people seem to think being online means that it is acceptable to interact via butchering the English language (or whatever language they're speaking).
If someone spoke in the broken language they typed in you'd think they were an idiot. You'd also be worried if they ended every sentence by laughing, as many people seem to replace a full-stop with "lol".