Completely random post here, but I've always liked making lists =P
Favorite to least favorite classes to play overall (class mechanics, lore, style, etc.)
1) Warlock (Duh)
2) Death Knight
*I'm leveling my Druid over my Priest, but only for Resto. Overall Priest is much cooler and I think would probably suit me better as a main. Discipline is OP and awesome at the same time. But Resto druid just have so many fun tools to use in BGs. HoTs (cmon I love DoTs, ya know I gotta love HoTs too), tree form is just cool, Stealth, and the overall different flavor of nature instead of holy. I still waffle between these two, but my brother is leveling his Priest on my account as we speak so I'll have a max level Priest at my disposal in time...
* Hunter used to be my main for ~half of vanilla. I don't know why it's so low. Maybe because everything I like about pet classes I get with the Warlock. That's not to say Hunters are bad, but IMO Warhammer handled "Hunters" better overall with more interesting mechanics by splitting the archetype into 2 classes: White Lion (Glaive wielding melee DPS class built extremely heavily on master/pet synergy, flanking, etc) & Shadow Warrior (pure marksman with no pet, different stances for kiting, sniping, etc.) After playing Warhammer the Hunter of all classes in WoW feels like so much of a mishmash.
* I haven't played a Shaman at all since BC so maybe the 3.2 tweaks (ESPECIALLY the totem changes) + generic overall tweaks might bump it up to ~Mage levels of fun. Overall though Shaman mechanics just aren't interesting. Funny how opinions can change because when I first got WoW out of the box I made an Orc Shaman...boy am I glad my RL friends convinced me to go Alliance when the character was only level 6.