I don't think balance ruined WoW, I think it was that Blizzard just kept dumbing it down.
Once they decided that every single player gets to do every single thing in the game it was ruined because you have to make everything so accessible. I remember killing C'Thun... we had a diagram on where to stand, responsibilities for which stalks to kill and in which order, hell, even running into the room wrong could wipe the raid. We were the only guild on the server who could clear AQ40. It was challenging and it was fun. Now 25 strangers can queue up in raid finder and just mow down every boss, usually without even knowing a strat. Just move out of the fire and cycle your 3 buttons.
All of the complexity has been relentlessly striped out of Wow. What's left is a game where you can logon, join a raid, kill the biggest boss in the game, and logoff all within an hour or so.
They took chess and turned it into tic-tac-toe.