Well the difficulty of the world also depends on what level you are. After a certain point, wizards on the same quest can be different levels. The difficulty of the worlds is probably created based on the level you are expected to be at that time. I am in Azteca now, and I have to say that Dragonspyre was definitely the hardest for me. This world may have been particularly difficult (and still is) because it was the final world back when the level cap was fifty.
Depends on what school you are, mostly. For my Life wizard all of them were pretty easy, but it took a while to complete them because battles take longer. For my Storm wizard, CL was nearly impossible.