WotLK was the sweet spot. From pvp to pve, from hardcore to casual. Patch 3.3.5 was icing on the cake.
Shifting away from WotLK formula was a huge mistake, WotLK literally pleased everybody and that's why it had over 12 million subscribers, I don't remember disliking anything, it simply was the best.
From the dialogue and the background story, the music, the atmosphere, villain and artwork. Everything fits perfectly. The gameplay was good and the community really was different in Wrath. There was no LFR. No cross-realm zones.
Naxx, ICC, Ulduar, TOC, the chopper, the mammoths, beautiful zones with amazing musical ambience and great quest lines. Wintergrasp was fun, VoA was a neat way to gear up, especially for alts.
The accessibility ICC had was amazing, you could pug it and still down things because Ventrilo etc.
I can honestly say I haven't even had half the amount of fun in the game as I did in wrath. Chain running dungeons back then was a blast. the 10 man raid was fun and manageable with a group of friends. It was a total blast. Pugs were amazing.
Class mechanics were a lot more fun, warriors in particular, stance dancing made them incredibly fun.
There was a lot of class uniqueness, not everyone had interrupts, not everyone had dispel protection, unfortunately Blizzard ruined everything with their "Bring the Player Not the Class" philosophy.