My favorite class of the day is the Hunter, mainly because I get to control both my main character and my pet at the same time. I love the dynamic of how these two entities fit together and do damage together so nicely. I have also played a warlock up to 30, but so far the dynamic of doing multiple things at once with demon master doesn't seem as interactive to me, though I suspect it may get better later on.
I love the multitasking involved with controlling two game entities with cooperative abilities at the same time, but some people hate it. One friend of mine wishes that hunters didn't have pets at all -- it feels too much to him.
One thing that I can't see myself getting into though, is multiboxing: running multiple WoW accounts on different computers and linking them up so you can control them at the same time. The prohibitive cost is the main reason I'm not interested, but also I like the sense behind the hunter class that the hunter and the pet were designed to work together as a single entity. I feel like multiboxing would only leave me doing less than would be possible if we actually had one real person playing each character.
What's your opinion on doing multiple things at once? Do you love such complexity, or do you prefer a simpler playstyle? Which class do you think has the most things going on at the same time?