30-40 definately isn't the hardest for horde. I guess not as many alliance do SM, but it's amazingly easy for horde to get to 40 once they start SM. As for running instances from 12-40, it's true that all of our instances up until level 40 are in out territory, except for maybe BFD, but nobody runs that anyway. Point is, it isn't like we can just level up to 40 simply by running instances.

I personally found 30-34 and 40-45 the hardest. Especially 40-45. God. Fuck 40-45. 40-45 fucking sucks. My characters will never be 40-45 ever again. Ever. Not even my currently level 21 mage. He will skip over levels 40-45 because I named him ChuckNorris. As a matter of fact he roundhoused Onyxia in the face last night and got 10 levels.