When I was very young I started playing WoW during patch 1.11, then cleared all current raid tiers during BC, then all current raid tiers during WotLK, and played for 2 months during Cata. I saw the state of the game, what was expected to come with the next few patches, and eventually the following expansion of MoP, and it was all very depressing. Honestly, the lore and execution of lore is what WoW has going for it. The story of WoW is a generally good one. However, Cata and MoP were godawful. As a huge supporter of WoW and Blizzard games in general, I have no problem saying that Blizzard messed up with Cata and MoP - there is a reason the subs dropped, what, almost 4 million? Saying the sub losses are due to the age of the game, or whatever else people are saying, is just ridiculous. I am very excited to see what Blizzard will do with WoD, there is tremendous potential with this story, and if things are done correctly, I honestly believe WoW will regain its former glory.
This is sort of anecdotal, but everyone I've talked to about WoW in real life, has said that they used to play, but that the game got bad after WotLK. Seems like I meet someone with this story on a weekly basis. Another thing people say is that all it'd take for them to return to WoW, is the return of the burning legion. The legion is the ultimate enemy in WoW - all problems trace back to it - and any expansion spent dealing with previously unheard of panda lore, or extremely poorly executed black dragonflight encounters, is seen as a waste of time by a lot of people.