I started WoW in '06. At first I was wary of it, because it was sorta a "new low" in regards of gaming nerds around where I lived - but I wasn't exactly conforming to society, and I found that I actually liked World of Warcraft. So I played, and had fun doing so.
When Burning Crusade was released, I was kinda upset. I wasn't a hardcore raider or anything, so from what I could tell the changes to the end-game content were all for the better - no 40-man raids, no AFK-type bosses (what I've been relayed) - but the inflation of loot was a little disappointing. It was too easy to get decked in epics. Having said that, Burning Crusade was my favourite expansion, and my favourite rendition of WoW overall. The fact that the classes weren't balanced is generally what I, as a layman, would call 'poor' game design - but it was that imbalance that made the classes unique, and the game fun.
Wrath of the Lich King aimed for balance. They succeeded. A bit too well. Instead of each class bringing something different to the table, every class were given 3-4 (may be wrong here) buffs, raid-wide, that did not stack with one another - a way to include every class into the raiding metagame. You no longer needed Enhancement Shamans for Windfury totems - or Paladins for Blessing of Wisdom. The unique buffs were replaced, and given to more classes. Ultimately, I didn't like it. I still played through as much content as I could bother to do, but it was no longer the old, clunky, poorly-designed fun World of Warcraft that I loved. It was too polished, too streamlined.
I barely played Cataclysm - but it wasn't my cup of tea. It was, like Wrath, polished to a blinding sheen. It was bland. The classes were distinct enough and it had its moments of difficulty, but making away with class-specific quests and unique buffs (which they did in WotLK) really... killed it for me. Compared to WoW four years ago, it was simply too easy.
To me, WoW as it was in its early days and WoW: Cataclysm are comparable to D&D 3.5 and D&D 4th. 3.5 was horrendously imbalanced, it struggled with some poor game mechanics, playing rogues and fighters were eventually pointless because Wizards and Clerics outdid them in everything (on a meta-powergaming level, at least). 4th tried to fix this by... making every single class play the same, more or less. Sure, each class has mechanical differences but ultimately, since every single class more or less has the same number of at-will powers, encounter powers, daily powers, and utility powers, healing is streamlined, ect., it... doesn't do anything for me. Of course, I don't play either of these anymore.
So yeah... might be my nostalgia-tinted glasses are glued to my face, but pre-expansion WoW and Burning Crusade were my two favourite renditions.