First of all I have to say that I hated Cataclysm a lot. Mists was a huge improvement, but I, unfortunately, haven't so much of free time as I had during Cataclysm's period. I see several reasons in this decline pattern:
1. They haven't fixed serious 25-man raiding. 25-man guilds was a huge force which has been pushing realms for raiding.
2. No recognizable villains. Wow had its peak in Illidan/Arthas times.
3. Too much of dailies.
4. People realized that Mists is not enough for compensating 2 years of game-time wasted during Cataclysm.
5. Still no balance. Seriously, they destroyed complex talent system, introduced simple one instead and still can't balance it enough.
And yeah, especially I want to thank you Blizzard for selling real-money items in subscription-based game..