There was a large discussion on various sites about what caused the drop. Many "Anti-LFR" players are dead set on LFR and the "over-casualization" of the game being the cause. Frankly, now that we have two patches under our belt, I have deemed that on a progression level this is closer on par with BC with the least casual expansion to date. LFR allowing more casual players to raid does not make up for the fact that everything that needs to be done in the game takes a lot longer. Blizzard even admitted in the same conference call that it was casuals "not feeling engaged".
Leveling? Drags out a bit the second time around. No ability to get flying mounts earlier on your alts. Heirlooms get updated, but only to 85 and required a lot of JP. Overall the process of leveling takes much longer then even Cataclysm on a level by level basis, and feels ages longer then Wrath.
Dungeons? They kept the promise of making the heroics easier, but instead gutted the ability for them to give rewards. The main currency gained from dungeons is JP, which has been all but abandoned in favor of VP in all facets of the game. VP items will no longer be downgraded to JP as far as Blizzard is concerned, meaning JP has pretty much died as anything other then heirloom and transmog currency. No new dungeons further adds to the problem. Instead they add scenarios, which would be great is the rewards were not a 5% possible drop from a bag at the end. It needs a consistent reward system.
Raids? LFR does allow casuals to finally raid, but long queue times and the fact each hall, depending on wipes, can take 30 minutes to an hour makes it still a pretty large investment on the part of the casual player. Even then, you could go through the entire event and only get 120-240g. The worst part? LFR still, for whatever reason, has weekly boss lockouts. You need a specific item off a specific boss? Better get lucky because if you fail you either wait a week or hope you have enough coins (which are also gated weekly) to keep trying the bonus rolls. No more ability to take a open weekend and just run dungeons all afternoon till you get that item you wanted.
Dailies? Dailies have always been seen as something fun for casual players to do. However, they still follow the same rules as any other thing in WoW that a mostly casual player is looking for. They need to be quick and painless. Some reputations like Cloud Serpent, Anglers, or Tillers are perfect. They are varied, they change each day, they have a little bit of story. Others like Golden Lotus and Klaxxi can be just painful to do. They go on for a long while, can't be done all at once, and often times involve mobs that are beefed up to be more threatening. Quick and painless is the way to go, and always has been. This is one of the reasons everyone hated the jousting dailies back in Wrath.
All in all it comes down to one thing. MoP feels like it's trying so hard to come out with fast patches but on the other side of the scale is attempting to drag out as much of the game as possible. There are no easy "catch-up" mechanics as of 5.2 and that combined with all the other things means that with each patch we make it even harder for someone that started a little late to join the party in a time frame that works for him.
That, most of all, is one of the big reasons I think a lot of people left. My understanding is that a lot of people that play in China do such out of internet cafes. These are people that pay by the minute/hour and need to be able to progress quickly in that time frame. Blizzard spent a lot of time attempting to drag out what players could do as much as possible, removing some convenience to do such, and there will come a time the player just does not want to be dragged on anymore. Even the horse will someday get tired of the carrot if he can never get a nibble.