They had consistent gains and several big jumps during TBC, and they threw that all away for a relatively tiny gain and an almost complete plateau in Wrath.
Wrath was where it all started to go wrong, I think people overestimate how good ICC was, because it was pretty average really. Ulduar was the best raid of WotLK and almost all of TBC's raids were better than Ulduar.
I won't tout about how good Vanilla WoW was because it wasn't that great, it was good, but they got it right in TBC and then decided they had too many completely dedicated players, they needed to open it up to casual flakers who'll drop the game just as quick as they picked it up.
T9 (that argent tourney shit) in wrath was so bad I left to play Aion throughout that entire content period, and it says something about your game when a player from day 1 and raider in a top 250 world guild would rather play fucking Aion of all things.
Came back for ICC, which was about 3 good bosses, and many, many mediocre (and worse) bosses. Got realm first 85 Undead, Rogue, and Horde and played through until Heroic Firelands and decided the game just wasn't for me anymore. It isn't even world of warcraft, it's a completely new, worse game, in a WoW wrapper.
Edit: On the topic of balancing, I never understood why they tried so many stupid and fruitless things like special resilience pvp gear, pvp power etc etc. That was never going to balance anything, all they end up doing is balancing a class for either pvp or pve and then completely fucking them over in the one they aren't balanced in (like my precious Rogue, so many times, being nerfed in to the ground because of PVP shit I couldn't care less about).
All they had to fucking do was make abilities work differently in pvp, that's it. Are rogues doing too much damage in pvp but they are just right in pve? then reduce the damage their abilities do while in a BG, Arena, or designated pvp area. Mages have too much CC for pvp but it's needed for pve? give some of their abilities a longer cooldown in a pvp setting. It really isn't that hard. Make pve and pvp separate, stop trying to balance every class against every class while also trying to balance them against every dungeon and raid boss in the entire game at the same time.