This is how you get from Night Elf territory (Teldrassil, Darkshore) to Human or Dwarven/Gnome territory (Elwynn Forest,Dun Morogh):
There is a portal west of the bank in Darnassus that takes you to the Night Elf port, Rut'theran Village. (The portal looks like a hazy pink tent under a tree.)
From there you can take a Boat to Stormwind. The village has three docks. The eastern dock remains unused, though a boat is permanently parked there. The western dock goes to the Exodar. The southern dock takes you toStormwind.
You can catch a free ride on the Deeprun Tram in the Dwarven District of Stormwind that will take you to Ironforge.
With the help of a friendly Mage to portal you between continents, or a warlock to summon you, you can quickly gain access to all four major cities without any zone-to-zone running. Portals to Ironforge, Stormwind, Theramore, Darnassus and Exodar are available to level 42 mages. This will allow you to learn weapon and trade skills at very low level. There's no longer a reagent needed for portal spells as of and thus requires no cost to the Mage, making it free to use a portal. While in Kalimdor, pick up the flightpaths at Rut'theran Village and Auberdine.(destroyed since , replacer: Lor'danel) While in the Eastern Kingdoms, pick up the flightpaths in Stormwind orIronforge (whichever one you find yourself in), and take the Deeprun Tram to get the other. Use your hearthstone to get home.
If playing a druid, it might make sense to set your hearthstone in the Eastern Kingdoms and rely on Teleport: Moonglade to get back to Kalimdor. This is particularly useful if you're questing on both continents at once.
The Fastest Possible Route
Teleportation. You may have a mage create a portal for you to Stormwind or Ironforge, or have a warlock summon you, but this requires that you have a fairly well leveled friend who is willing to put aside a few minutes for you, or a fair amount of money to make it worth someone's time.
or alternatively any class can use the meeting stone located at the stockade instance in stormwind city, so long as you meet the level requirements and min. 2 party members are at the meeting stone. you can then take the deeprun tram to ironforge.