I agree on the line spacing and punctuation, plus using capital letters to lead off sentences is good. I can forgive minor spelling problems. But when I see someone rattle on in all lower case with no sentence structure, I picture a lazy mumbling idiot. Even though they could be an intelligent, perfectly understandable person, and it may have been right in their head when they started typing, any logic is lost. I don't work to hard to convert it back to understandable conversation, and will move on.
Sometimes it seems like the people coming here for help with a mechanical/operation problem are the worst, and that is a heck of a way to make a first post asking for help. When I ask for help, I try to make it as clear as I can.
Ironically, my spelling has gotten worse with the darn autocorrect that takes over in most browsers now. Usually it replaces words I mean to type with something else (especially complex words). If I don't catch it, it's garbage. I have to spend more time reviewing and correcting my posts than before.