They changed and removed too many game play systems from BC/WotLK, failed to improve upon what they already had (like making the talent trees and professions more interesting), refused to add systems that people enjoy (housing would be a classic example), left dead servers dead for YEARS, and automated far too community related systems (such that people did not need to form bonds to complete content). I hope they're not surprised with this result. They ruined their own MMO, essentially by making it a single player game with other people in it.
Lore, story, humor, and art-wise, WoW is a great product. However when compared to Everquest II, another game it's own age, it's pretty clear how much pooch screwing has gone on at Blizzard over the years. EQII has more game play systems and "stuff to DO" than any other MMO, yet Sony Online Entertainment sure as heck didn't have as much money to spend or as many as employees as Blizzard (and it shows in how god awful the EQ2 engine is). But bless their hearts, the SOE devs have added a staggering amount of content of all types to EQ since it's launch, on a reasonably frequent basis. The same cannot be said about Blizzard with WoW and I think that plays a larger role in its declining popularity than people might think.
WoW is a very one dimensional game for an MMO. Not a good thing, unless you're looking to have a small community that consists entirely of people who enjoy exactly the same style of game play.
Ps. Most other MMOs have more variety of game play than WoW. I used EQ2, because it's the same age as WoW and the polar opposite in terms of content.