Online, discussions of grammar tend to display confusion about what the subject is, and the usual admixture of rubbish and emotion does not help.
There is, of course, the confusion between grammar as grammarians and linguists discuss it technically, and spelling and punctuation. But other, unstated meanings are often involved.
A post by Lucy Ferriss at Lingua Franca, "Grammar: The Movie," identifies some of the additional meanings that surface in a new documentary.
Spelling errors: If you write it's for its in your cover letter or resume, or confuse there/their/they're, you're probably not going to get the job. But these are merely spelling errors, as likely the result of carelessness as ignorance. Of course, they're obvious, so easy to spot that even a manager can see them, but they are still trivial.
Bad writing: Lord knows there is plenty of slack, inexpert, and impenetrable writing to be found, but that is not a problem for grammarians to address. Academic writing, for example, is notoriously wordy and opaque, but it is usually grammatical.
Pedagogy: The complaint that grammar is not much taught in the schools any longer is true, but the teaching was not all that effective when it did occur. The traditional schoolroom grammar is a jury-rigged structure of terms borrowed from the classical languages combined with oversimplifications, and the analytical English grammar that linguists have developed over the past few decades is virtually unknown outside linguistic circles.
In the three weeks that I can afford to spend on grammar and usage in my editing class, I rely on the traditional terminology, because scraps of that are what my students know. They would choke on the Pullum-Huddleston Student's Introduction to English Grammar, even assuming that I had the time to spend on it.
(If there is effective pedagogy out there, perhaps in ESL texts, let me know.)
Standard vs. non-standard English: Many people make the mistake of concluding that standard written English is correct English, all other variants being ungrammatical. That's just not so. We don't go in for double negation in standard English today, for example, but it was common, understood, and grammatical for Chaucer. And if someone says, "It don't make me no never mind," I comprehend immediately that the negatives intensify, without having to count them to see whether the result is positive or negative.
It would be helpful for people to acknowledge the whole range of registers in modern English, gauging which is appropriate for speaker/writer, subject, and audience, rather than declaring variants wrong.
Literacy: The standard/non-standard issue points to the cultural and political attitudes that are almost always present but seldom articulated.