Excuse my French...
Bon appétit. The rest of the world--especially the rest of the anglophone world--loves to use this expression as a signal to dig in and enjoy the meal, but in well-mannered French circles it's a term better heard than said. Or best not said at all. If someone is wished bon appétit, then he should simply nod or say merci, but certainly not replicate the infraction by responding bon appétit in turn. It's not a question of being rude, mind you, but an attitude that if at all, the expression is for waiters to say, not diners.
©2014 P.B. Lecron