Although I try to make my posts free of spelling and grammar errors as well as lucid and readable, I don't automatically ignore imperfect posts, and small mistakes don't bother me at all. I would certainly never criticize the spelling, grammar, and sentence structure of someone else's contributions unless they had provoked me by criticizing mine. Hasn't come up yet.
I personally believe individual spelling performance is either genetic (something to do with pattern recognition?...) or perhaps the result of very early educational experience or technique. I have no proof for this belief, only the fact that I have known many well-educated, intelligent, and well-read people who just can't spell worth a darn.
As for writing coherently, who knows. Either you have it or you don't. Back in college I took a class where we reviewed/critiqued each other's papers. I saw a few that were nearly unreadable - garbled structures, massive run-on sentences, and so on - from intelligent, educated people. On the other hand, I see posts here from people who by their own admission barely made it out of high school that are clear, concise, and stylistically smooth.
It seems that being able to think or speak clearly does not always result in being able to write clearly. It's not safe to assume that garbled and disjointed posts on an internet forum are necessarily the product of an ignorant or stupid person, although I admit it can be tiresome to wade through reams of near-gibberish to divine the intent of the writer.
I do get extremely irritated by those who point out minor spelling/grammar mistakes. At best it is irrelevant nit-picking or compulsiveness, but it's often employed by those who have nothing else to offer. See:
http://redwing.hutman.net/~mreed/warriorshtm/grammarian.htm
My problem? I'm a little long-winded.