One reason I left was that I was spending far too much time on the game. After four months of playing I was logging in for three to four hours every day and spending hours more browsing internet forums, blogs and help sites relating to the game. Although I have been a gamer for many years this was my first MMORPG and I was ultimately appalled by the amount of time involved. This was time that was robbed from family, from real life activities at home and in work and it was time robbed from enjoying other games. To make matters worse I realised that even though this was an unacceptable time commitment for me it still only put me in the casual player category - far below the commitment of dedicated end game raiders.
Another less obvious but perhaps more lingering reason was my terrible disillusionment at what WOW end-game does to Guilds. Like many newbie MMORPGers I played the game for some time before joining a Guild. Being invited into a friendly casual guild was a revelation opening up new dimensions to my gaming experience. As I have said this guild was friendly and casually oriented. Guild chat was used for gossip. High level members helped out lower levels. The Guild organised regular events like treasure hunts which were open to all. Guild meetings often ended in a drunken Tavern crawl. Even though real life commitments often prevented me from contributing fully to guild events I really enjoyed the friendliness of it all.
As the server matured more players began to reach the level cap and in order to continue their advancement they were forced to look seriously at their play style and at their guild. If these players wanted to continue advancing they would have to become a lot less casual and that meant either the guild must change or they must abandon it for a more serious "end game" focussed guild. My internet reading convinced me that this problem wasn't unique to our guild or to our server. I came across many many stories of friendly fun guilds who failed to survive the transition to end-game.
Perhaps the biggest eye opener for me was when the leader of the largest and most prominent guild on the server publicly announced his resignation so that he could take a humble foot soldier position in a raiding guild. The large guild was of course a casual guild and despite its size it was unable to withstand the transition to end-game. According to Warcraft Realms that guild has only one active member left today. The fact is that WOW's end game destroys guilds. In particular it destroys friendly casual guilds, the kind of guild I want to play with. I still haven't forgiven WOW for that.