One of the reasons that online discussions of grammar are either sterile or acrimonious is that some participants are not really interested in grammar, but rather come to the discussion seeking validation of their class status.
The first indication lies in the sorts of things that people complain about: spelling mistakes like it's/its, errant apostrophes in menus and signage, "irregardless is not a word," casual usages in speech. The very triviality of the examples, combined with the vehemence of expression, suggests that something deeper is going on.
This becomes nakedly apparent when the complainer calls people who veer from the conventions of standard written English "idiots" or dismisses "people without much class or education." The "I judge you when you use poor grammar" crowd appears to be less interested in improving the speech and writing of others than in demonstrating its own superior education and social class, less interested in correcting error than in acquiring affirmation of their self-congratulation.
Exchanges grow heated when someone dares to challenge some schoolroom dogma with an explanation that English grammar and usage are more complex—that, for example, while that generally refers to animals an inanimate objects, it has always referred to human beings in certain contexts, such as unnamed persons or groups of persons.
The "I judge you" caucus cringes at "between you and I" and "me and Janet went to the movies," but examination by linguists of how the language actually works shows that some of these coordinate combinations are well established patterns. Elsewhere, Barrie England quotes a citation from the Oxford English Dictionary's entry on I: "Used for the objective case after a verb or preposition when separated from the governing word by other words (esp. in coordinate constructions with another pronoun and 'and')" [emphasis added]. Such combinations, though they are not thought appropriate for published prose, are common in speech and do little or no harm there. These are merely differences in register.
But to the complainers, the concept of register appears to be abhorrent, because it undermines their belief that the constricted version of standard written English they learned in school is English, all else jibber-jabber.
(I speculate that they turn to grammar to demonstrate their social superiority because they lack access to the typical marks of elevated status: large houses vulgarly furnished, environmentally obnoxious vehicles, clothing stamped with designer labels, and all the other marks of the middle class striving to ape the gentry.)
It is actually possible to learn something in these online discussions, since truly knowledgeable people do contibute, with examples and citations of evidence. But you have to work through a great deal of noise.
A personal note: It is doubtful that anyone has entirely clean hands here. We all make private judgments about language based on social and class markers. I grew up in Appalachia, and when my parents visited me at college, they were apprehensive that people there would think them "country." (That was in urbane, cosmopolitan East Lansing, Michigan.) They indeed had regional accents, though not the most strongly marked ones, and I, though I did not have much of a regional accent even in childhood, did take trouble to clean up a few diphthongs in adolescence.
Probably just as well to keep private judgments private.