Higher level Trainers (mostly Engineering) and Vendors.
Artisan Cooking quest.
A neutral Auction House.
Fast access to the Shimmering Flats.
Hanging out (or fighting) with the Horde.
Another flight path!
This is how to get from the Dwarf city of Ironforge to the Neutral city of Gadgetzan near the southern tip of Kalimdor:
From Ironforge, run down the ramp and take the road east past Gol'Bolar Quarry[ and take the north road at the fork past Helm's Bed Lake.
Go through the north tunnel to Loch Modan and go south from the tunnel exit until you hit a fork and take a sharp turn left to the north through another tunnel.
You may run into some orcs after you exit the tunnel, so just try to run away from them if you are too low level. They wander on either side of the road. There are a couple of more tunnels until you reach the Wetlands.
When you exit the tunnel to Wetlands, watch out for spiders and crocolisks if you are below level 20. Follow the road as it curves north and back west to Menethil Harbor along the hills to the left.
Once at Menethil Harbor, go to the southern pier and take the boat to Theramore Isle.
From Theramore Isle swim south off the pier along the eastern coast until you get to Tanaris.
Eventually you should reach Steamwheedle Port on the northeastern coast of Tanaris.
Once in Tanaris, run west on the road to Gadgetzan. Watch out for the hyenas as they will probably kill you in 1 or 2 hits if you are below level 30 or so. The hyenas don't get too close to the road, so you should be able to avoid them if you have a sharp eye.