The threat is ambiguous, and can result in many different outcomes. Egging, toilet-papering, or bags full of dog shit are commonly expected, but graffiti, and vandalism, and other criminal acts may be committed.
Larry David: "No, there was no threat, except for the "trick" threat."
Police Officer #2:"What's the trick threat?"
Larry David: The "Trick or Treat". No treat; trick. It's a threat! How far can you take these tricks?"
Police Officer #1: "If it was any other night sir-"
Larry David: "TRICK OR TREAT -- BANG, BANG!"