I will say with ease that Wrath was by far my favorite Expansion. Wintergrasp was an amazing PvP environment even if it took countless patches to figure out how to balance out Tenacity and I never got my gnomish flying bombers.
I remember the instant drops when Occulus was the heroic and how people wouldn't touch it until they let the drakes scale with iLvL after I think Ulduar came out. Trying to do all the new achievements that were introduced and finding yourself trying to convince 25 people to move under Bubbles in Malygos. Now fun it was to slowly see the Coliseum in Icecrown getting built, all the quest before hand to set it up.
The calming soothing music of Grizzly hills as you fished up salmon for the feast, or the awesome lore of the Nerubians, Frost Troll Empire, and the Vykrul. Getting Drakes as mounts for the first time (even if one already had their netherdrake) being able to get the Bronze Drake and how much of a pain in the dick that was when people were all still in heroic gear or T6.5.
Running Naxx and reliving the old Vanilla Memories, or trying Sarth 3 Drake and it being tough because you didn't have the Raid Wide DPS to attempt it. Finally getting to see the Lich King and all his minions, getting ToC. THe chance to Fight Algalon the observer and the insane difficulty of the hard mode fights where you'd have to take a Feral Druid or an Unholy DK to tank. Timing My Mangle with it's 12 Second Uptime to keep Rip and Rake up. Stacking Armor pen because it was the thing to have, and so you'd wonder around with all these Red Armor Penetration Gems, farm for Grim Toll in Naxx or pray that you got that trinket off I think... Loki maybe I can't remember.
Then finally the Lich King itself, as it was slowly unlocked. Of course by that time I had left my Raiding Guild for a quieter more relaxed environment which of course mean we even had trouble on Marrowgar, let's not even talk about Blood Queen or others.
Figuring out who the Ulduar Legendary would go to, or putting together Shadowmourne and seeing people bring it into a battleground and just stand as far away as possible.
For me the Snowy Plains of the North was where the heart of my WoW career was.