For me it was the dying of my realm going from raiding 10-15 hours 6 days a week at start to once or twice a week cause we couldnt get enough players, and the thing that there was nothing for me to do as a solo player, I was exalted with all the factions and had made about 200k gold ( gifted like 100k at least to guild bank and 50 k to my brother) When the changes came to the lowermost zone I didnt find those quests any fun at all so I didnt do them and as I said my raid was kind of dying and I didnt want to change guild. So i decided it was time to quit again ... ( first time i quit was (vanilla - Wrath i played) then i quit played again in wraith ( quit again played again in wrath, and then quit and played at start of cata then quit again and started at beginning of mop)