It would depend on the error and whether or not the error changed their point or simply made it difficult to understand. If they said, "I hate guppies!", but meant, "I don't hate guppies!", it makes a bit of a difference. If it's style-related, i wouldn't mention it ("Dear Mr. Joyce, Your style of writing with run-on sentences and invented words is not suitable for publication...."). If the entire post of 12,000 words is presented in one long paragraph, I'd post whatever reply I had and I'd add a, "Hey, you might want to consider breaking your post up into paragraphs to make it easier to read." addendum at the end.
Grammar mistakes and typos I tend to ignore. People generally won't appreciate a "heads-up" PM about them and if you post about them you'll start a flame-war about who is/isn't the "grammar/spelling police". (Quietly grits her teeth over "its/it's", "your/you're", "they're/their/there" and "to/too/two".)