To start of, I'll say this: I have not looked at any other information other than what I've been hearing through sources such as The Instance or MMO-Champion and using logic to come to my conclusions. I am in no saying that these reasons are the true, exact causes for the recent decline in subscribers. I always welcome new and accurate information. I ask that only that any ensuing discussion remain civil and that no one feeds the trolls.
1. Blizzard, by catering more towards casual gamers, inevitably sealed their own fate. You may be asking "How?" right about now. Casual gamers are looking for fun that doesn't take hours to grind for, a game that can be picked up for 30 minutes to an hour at a time and make significant progress. While you may be able to do that with WoW now, it feels (for me at least) like to succeed big time would take a few hours a day, time that I simply do not have available. I had been running the same content for about a month, with one random dungeon a night, yet I didn't feel like I was progressing at all. And that's when the game gets boring. And when casual gamers are bored, there is no hesitation to stop playing the game and moving on to another one, such as Skyrim (where progression feels faster and more organic). Casual gamers are a fickle crowd, and with the rise of several games that demand more of our time (lookin' at you Skyrim, Minecraft), we will stop putting money into a game we don't play.
2. This next point also is about casual gamers, however in a different context. By serving casual gamers, who feels shafted? The hardcore, nostalgic players who see "their" game falling to pieces around them. And that is a valid feeling. I've been saying that since 3.2, Blizzard seemed to be disregarding those hardcore players who want to have epic, difficult fights. I subscribe to Blizz' idea that as many people as possible should see your content, I also believe, however, that you should be rewarded proportional to the amount of time spent in dungeons and raids. A way Blizz could've done that is by making the 10-man raids the raid finder difficulty, 25-mans the medium difficulty for your generic raid guild and bring back 40-mans for the baddest of the bad-assest guilds. Or the raids of an X.1 patch could've been nerfed in the X.2 patch. But something should have been done for the top 5% of the game.
3. Price. 15$ a month is a lot of money. For that amount I want to be able to play at least 10 hours a month. But a lot of people can't play that much. And with games like CoD/Battlefield and Skyrim, which require only a one-time 60$ fee, provide 100's of hours entertainment. WoW can't compete against that to casual gamers. If Blizz had slowly dropped the price as their subscriber count rose, many people might have started playing (I won't go so far as to say that WoW should've gone free-to-play). I have a lot of friends who would've played WoW, but the price chased them away. Perhaps they would've been willing to play if the price had been 12$, 10$ or even 8$, but something a little lower.
As I said, if you would like to discuss these points, I am open to conversation. Just make sure it remains civil. I am not by any means saying that these points are the truth and need to be accepted by all. I thank you for reading this post.