No-fly zones are areas in Northrend where flying is not possible. These include the city of Dalaran (except the Krasus' Landing); and the whole PvP zone of Wintergrasp. In these you can't mount on any flying mount or shift into Flight Form. When entering one of these by flying, you will get a warning and after a few seconds will be forcefully dismounted with a parachute-like buff to prevent dying from falling. The parachute lasts until you hit the ground.