Prepositions at the End. Along with split infinitives , a favorite bugbear of the traditionalists. A sentence becomes unnecessarily obscure when it's filled with from whom s and with which es. Prescriptive versus Descriptive Grammars. They're doing important work, not least by arguing that no language or dialect is inherently better than any other.
Many prescriptive guides are grievously ill informed.
Sometimes, though, I enjoy picking fights with those linguists, usually amateur, who try to crowd prescription out of the market altogether. The worst are hypocrites who, after attacking the very idea of rules, go on to prescribe their own, usually the opposite of whatever the traditionalists say. These dullards forget that words are used in social situations, and that even if something isn't inherently good or evil, it might still have a good or bad effect on your audience. I happen to know for a fact that God doesn't care whether you split infinitives. But some people do, and that's a simple fact that no statistical table will change.