Player Volition of Burning Blade server posed this question on the official forums recently: what is the hardest class to play to its fullest potential? The question got further refined to: what class is easiest to play, but hardest to play great?
Druid, Shaman and Hunter came out ahead in the thread as difficult to master. Druids and Shaman for their myriad options and playstyles. Hunter for learning the exact timing of shot rotation and pet threat.
I can attest to the Druid part, especially if you are playing the secondary (tank/healer/dps) in a 5 man. You have to know how to play many classes at the same time and you have to know when to be using what form. Mage gets aggro. Do you bear form and charge. hoping to get it back or stand back and heal hoping the Main Tank can snap aggro the mob? For me, that's the fun of the class.
What do you guys think? What class takes the most skill to make the most of its talents?