I generally am annoyed by poor spelling and grammar only if the person is purposefully doing it to look “cool” such as leet, excessive chatspeak, tYpInG LyKe ThIs, and the lot.
I find purposeful mangling of the written word to be annoying and rude. First thing, it says that the person does not care about the reader: their exercising of freedom of expression is more important than ease of reading (I have been told exactly that). There are people on primarily English speaking boards who are dyslexic like myself and have varying degrees of difficulty deciphering such things. There are also people whose main language is not English. It has to be even more difficult for them to decipher these things than it is for me.
Occasionally I say something to people who do this but usually just ignore them.
However, if the person is making their best effort but still make lots of errors it does not bother me. Some people are dyslexic (like myself) or have other learning disorders, English is not their first language, or they did not have decent English teachers in school.
If they fall under the “making their best effort” crowd, if I know that the person would welcome constructive criticism, I often will say something. However, I do that only if I know the person and know that they will not be offended.
If the question or answer from people who are trying their best is indecipherable to me and I want to know what they said, I will ask them to please rephrase it, telling them that I really want to know what they said but I just cannot make it out.
Personally, I have no problem with someone correcting my spelling, grammar, or improper use of a word as long as it is done to be constructive rather than using the “correction” as an insult. If they do it constructively, I thank them. If it is as an insult, I react a little less pleasantly.
“Are we snooty and “snarky” about grammar and spelling.”
—more so than most sites. I find questions and answers much easier for me to read here because of it.
“Are we justified in responding to the errors?”
—as long as it is not used as an insult.
“Does it raise or maintain the level of discourse.”
—raises the level.
“What’s wrong with setting and expecting an effort towards maintaining high standards?”
—I have no problem with this as long as people’s efforts are recognized and difficulties are taken into consideration. I do not believe that someone should be discouraged from participation if they are doing their best and believe that insults are unnecessary.