Being a mediocre writer, I don't know what it means to write like you talk, but I know I've noticed a strong correlation between how ornate one's language is and how little one knows what they're talking about. The people who know the most use the simplest words, and if someone uses complicated language, they're either trying to deceive or to hide the weaknesses of their argument.

This only goes for specific cases, of course. E.g. it probably applies more to business language than to novels.