No question, it was by far my favorite as well, and I'm surprised Stickney only went with five reasons: There are TONS of reasons to love WotLK!

DUNGEON FINDER
Overhauled in 3.3.0 to allow cross-realm queuing that instantly ported gamers to the dungeon. This changed the game in an immediate and comprehensive way. Depending on your point of view it either made the game significantly more accessible or "ruined" the sense of server community.

It allowed nearly every gamer that was interested quick access to Heroics. With Heroics running 30 minutes or less in most cases it meant that casual gamers could gear up for entry-level raids, particularly since Emblems of Heroism and their successors could be used to purchase extremely powerful gear.

Not only that, the Dungeon Finder worked for lower-level dungeons. This allowed leveling gamers to actually experience dungeons like Razorfen Kraul and Blackrock Depths in level-appropriate groups.

Finally, in later iterations it allowed instant access to holiday bosses like Coren Direbrew, making what was once an arduous trek to Blackrock Depths and an awkward rotation of daily quests quick and easy.

It was, in my opinion, the most important development in Wrath of the Lich King, if not the game.

VEHICLES
For better or worse, from riding a Mammoth or a Siege Engine in Borean Tundra, to riding a "rail-shooter" horse in Grizzly Hills, to the back of a Storm Giant in Zul'Drak (Storm King's Vengeance, how I loathe thee *shakes fists*), riding an abomination construct in Icecrown, and even blasting through Ulduar, vehicles were everywhere in Wrath of the Lich King.

It added variety to gameplay and some unique quests that would otherwise have not been possible without a vehicle interface. It was occasionally buggy and frustrating (*ahem* Oculus dragons), but the change of pace was - for the most part - welcome.

HEIRLOOM GEAR
For better or worse, Blizzard gave players with level-capped characters the opportunity to accelerate the leveling process for their alts. Since Heirloom gear was Bind on Account, it also allowed players to gear low-level alts on the opposite faction where they might otherwise have no resources in terms of gold and materials. This encouraged many players to go back and try out new races and class combinations where the level process might have previously appeared daunting (remember, this is pre-Cataclysm, and low-level questing areas had yet to be overhauled).

WINTERGRASP
or "How I learned to stop worrying and love Siege Engines"
World PvP changed dramatically with the introduction of Wintergrasp. Blizzard's prior attempts at World PvP - Eastern Plagueland Towers, Hellfire Peninsula Towers, Nagrand - had mixed results. Wintergrasp allowed gamers from a particular server to gather for a world PvP event on a regular schedule.

Despite balancing issues with regards to Tenacity, the objective-based assault on Wintergrasp Keep was, for the most part, successful in getting same-server players together for PvP.

DUAL-SPEC
I know you already mentioned it, Shinae, but this was a Godsend for anyone looking to both raid and PvP, or heal and tank, or tank and DPS, or whatever other combination of gameplay a player could want. Sure, you could go back to the trainer and unlearn your talents manually for a fee, but that's a long trek to make when you just had your tank healer leave due to a personal emergency! The amount of options afforded by the system made raid composition much more flexible, and allowed some players previously pigeonholed into a particular gameplay style on-the-fly access to new roles.