Very interesting thread.
I can only speak for myself so I try to address what I disliked about MoP:
First the whole storyline and continent:
For me WoW has always been abou our main factions and races but what they basically did with MoP is forgetting about that and adding Pandas everywhere. Thw whole addon was basically centered on those Pandas, you always had to deal with them and to be honest, I nevr liked them.
They were cool as a sidekick in TFT but they should have never become so important and especially not a whole addon catered to them.
Secondly:
Everything has been dumbed down alot. With the introduction of LFD it was obvious that hard heroicdungeons will not play out. If they are so hard that you will wipe a few times with a premade group of friends, they will be impossible with a random group thus the only way to not let the feature die is to reduce difficulty of dungeons since the QQ in Cata was extreme.
Now we got LFR aswell making it so easy to see the content that many players, including me, don't feel the urge to do the "real" raids because I have already seen everything and it isn't enough for me to play the whole raid again with a mob that has one attack more.
So while I had toraid and progress slowly through the content in WotLK and Cata, I didn't have to in MoP I queued for LFR and saw the whole raid content in a few hours.
Thirdly:
Class restrictions
With Caa it began that we have been forced to skill into one tree and not being able to hybrid or to skill and customize our character as we please.
There was no longer experimenting, there was no longer a way to get around a certain playstyle for a class if you didn't like it.
It dumbed down char progression and made it boring. With MoP they even went past that. We don't have a choice anymore and leveling up is not interesting anymore. 1 Telent point each 15 levels is not interesting and not char progression. We even lost spec specific spells even if if it was not optimal as a frostmage to use firebolt, it was still nice to have it.
It is even worse with Warriors, who lost every shield specific skill in Fury and Arms, no more shieldblock and revege spam when turteling, no more shield slam dispell and so on.
Classes even lost signature skills they had since the begining and defined the class, for exemple Intercept has been completely removed.
This leads me to:
Fourthly:
Alts are not fun anymore.
There is nothing fun left in leveling an alt. Cata has basically destroyed alt leveling with teh revomp of teh zones. The zones are so linear and so boring that it is just not fun to level anymore. Also as a veteran player the massive revamp disconected me from the roots of the game, destroyed my nostalgia that drove me to play another alt, just to see the old zones again I "grew up" with.
his coupled with the removal of talent trees and only one choice every 15 levels made it evenmore boring and unrewarding.
So my conclusion to the massive drop in subs is basically this:
The subs that have been lost are probably mostly veteran players who just don't feel "home" in this game anymore.
Basically everything that made WoW interesting for them has been changed, dumped down or replaced.
For me personally the massive changes in the game systems, world and classes that took place with the beginning of Cata or end of Wotlk are the reasons.
To some extend I think Ghostcrawler can be blaimed, not because his changes were bad but because they drove WoW into a completely different direction than before making it basically a completely different, easier, game that is no longer the game I used to love.