Foreword: This exploit requires a diminutive stature, and as such is not applicable to horde as far as I know. I did this as a dwarf, presumably it will work with gnomes too, however it does NOT work with humans! (hence the assumption of height)
Alright, so first you head over to krasus landing. Look to your left and fly up to the ledge, make sure you land so your mount is running. (your collision box might be too big otherwise)
Then head to the spot in the picture below, and go forward. You should clip through the wall.
From there you can fly to just about anywhere, but becareful there are a handful of spots that have NO way out other than hearthing (auto-unstuck marks the area for the dev's to look at, so don't use it!)
In this pic you can see an overview of the area Northwest of the landing, the mass of water is the black market (area across from the sewers inn)
I was using a normal flying mount, so I couldn't quite reach the arena area, but with an epic flying mount you will definitely be able too. Just be careful about where you are flying, the walls do not have textures on your side!
Pics of the areas I got stuck:
And the croc!
by tiptopz