To get off Northrend, all you need to do is reverse the order you used to get there. You can take the boat/zeppelin from Borean Tundra to get to Stormwind or Orgrimmar. The boat/zeppelin in Howling Fjord will take you to the Wetlands or Undercity respectively. There is no direct route from Northrend to Kalimdor for the Alliance (Stormwind and Wetlands are located in the Eastern Kingdoms)