Druids are out of the question, eh? 
  
Played a warlock to level 45 back in vanilla days, but I don't really remember much of it. Pretty boring at times, but strong DPS and strong at turning ganks around. 
  
Can't really give any insight on shamans or paladins... except that paladin is the most boring class of them all. Oh, and I will never give them an opportunity to tank or heal an instance. 
  
Leveling a warrior is the most masochistic thing you can do: pull more than one mob and you're done. Things get a little easier when you get your Whirlwind Axe and a lot easier in the end. Able to pull off some brutal dmg. Recently played one to level 35 on old Nost, but got fed up with it. 
  
Mages are very fun to play, extremely strong in both PvE and PvP and very desirable in dungeons due to their CC. A pain in the попа for most classes. 
  
Priests... Duh, BORED! Next please. 
  
Hunter is the easiest class to level, but has an extremely high skill ceiling. You can suck hard (I mean HARD) if you are a bit confused with your class and rotation etc. Mastering the hunter is difficult, but very rewarding. Probably rolling this one. 
  
IMHO rogue is the most fun class to play due to its decent difficulty. That said, rogues are one of the easiest kills out there if they don't know how to play. Same goes for huntards - skilled hunters are a pain in the попа though. 
  
Eh... This post probably wasn't informative at all. Sorry for that, move along.