As a clever man once said "I begrudgingly felt that world of warcraft is about as good as mmo's are ever going to get" and I have to agree. Whilst I haven't played WoW for quite some time, I did recently go back thanks to a friend sending me a free trial for giggles. It was at this point (well approximatley 30mins in) that I remembered the reason I left in the first place. I left because I started playing before any expansion packs existed and my enthusiasm whaned in direct proportion to expansion packs being released, it just became too easy. In being made more accessible for the masses (a sound business strategy, more people = more money afterall) the challenge was slowly bled out of it until its something a five year old can play and do well in (something I've witnessed a scary number of times), this was reinforced by the achievements system, the gentle coo's of "well done" for doing meaningless things.
Yes I did play for a long time and I even tried to justify to myself the choices made by Blizzard, but when I caught myself tripping over achievements and pants on head retarded players every 6 seconds I knew the honeymoon was over for me. So to sum up, WoW was a good game but I left because as challenge drops the number of obnoxious players increases and why would you want to pay money to be insulted?
(Incidentally this is something WoW has in common with Xbox live...)
Oh and incase your all wondering yes I had a scary amount of level 80's and 70's, read into that what you will.