Vanilla was fun because of the newness of the game. Everything was disorganized and yet epic at the same time. Sure, it was mostly rock/paper/scissors, but it worked and was debatably more balanced than what we have now. Rogs would slaughter cloth, but get stomped by warriors, who get lit up by cloth (unless there's a hpal pocket healing...then it was just roflstomp ALL the things). I'd even say that burst was better than current as well. Epic AB matches with feral/rog engineers ninja-capping nodes. Insane WSG matches that lasted hours, actually porting OUT to get MORE free action potions/health pots and queueing back up. And the day-long AVs with Ivus and tickery back-capping.
Not to mention, the COMMUNITY in Vanilla was the best part, as you KNEW who you were fighting. Sure, ppl could duck queues, but the majority of the teams you faced when doing your grind to GM/HWL were the same opposite faction teams you knew and loved/hated/respected. There was no anonymity, and you made a name for yourself and your team, good or bad.
BC was great for other reasons, primarily making "off-specs" viable in most levels of play. Sure, you had the perennial classes/specs that fared well, but TBC allowed for some creative use of class abilities/specs to be successful. And what's more, is that it did so without relying on utter homogenation to do so. CC or interrupts for paladins? Nah, bro you got a stun. Interrupt/silence/stun on warriors? Better be good at stance dancing to keep on your target and either slap on a shield for bash or swap zerk for pummel. Feral interrupt? Bear charge. The lack of homogeneity kept a certain skill cap that has since been watered down and all but disappeared. While it can be said that arena was the "beginning of the end" in terms of balancing, since it tried to make this game something it cannot/should not be, it was also some of the most fun I've had in a PVP environment.