No-fly zones are areas in Northrend where flying is not possible. These include the city of Dalaran (except Krasus' Landing) and the whole PvP zone of Wintergrasp. In these areas, you cannot 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.
Kalimdor, Eastern Kingdoms, and Northrend (before you get Cold Weather Flying) act similarly to no-fly zones in that you cannot mount on any flying mount or shift into Flight Form, but you will not be forcibly dismounted, since there is no way to enter these areas while flying.