Q. If you’re replying to someone and want to say thank you, would you write “Thank you John” or “Thank you, John”?
A. If you happen to know that the object of your gratitude is a stickler for vocative commas, then write “Thank you, John.” Otherwise (and unless the context is formal), feel free to leave the comma out, a choice that suggests a certain degree of spontaneity and a friendly disregard for the rules. But if you’re an editor of any kind, use the comma regardless of what anyone might think; you have your reputation to maintain. For more on these commas, see CMOS 6.53.