Disclaimer: This opinion is coming from a WoW Casual (Don't raid anymore) Vet from all the way back from Vanilla Open Beta, up till hitting 85 in Cata. I've been well versed in WoW Lore ever since WC2 and the novels. Since then, I've moved on to RIFT and EVE while awaiting SWTOR.
Honestly, for me, it's just that I'm sick of WoW. It's not that I am tired of the game mechanics either, it's Blizzard, the Azerothian setting, the graphics style. The core mechanics of the game (hotkey skills, talent system, mounts, auction house, crafting system, ect) are all fine and good, but it's the aesthetics and 'theme' of the game that has worn me down. In part, that is why I play Rift right now. The mechanics are essentially the same, but it has enough of a unique spin on the world, atmosphere, and class system that I am truly enjoying the hell out of it.
Blizzard's biggest thing was that WoW was polished enough to keep it's players in the game, while also releasing addons and content updates at enough intervals that they screwed with other game's launch cycles. Now, after 6-7 years, people like me are just getting tired of Azeroth and are either ducking out of the genre altogether, or are looking for the next best thing to fit the mold.
That said, I don't think you'll be seeing WoW shutting down their servers anytime soon. They are still at 11.1 Million accounts (how many are paying, we don't know, but still), a figure that ANY other game would give both of their nuts to come close to reaching. Unless Blizzard takes some very drastic steps to anger their existing player-base, that number will remain high for quite a long time. What it does give, however, is a chance for other games like SWTOR and GW2 to launch and try and retain their initial player-base. As far as I know, Rift has actually held on rather well to their player base, especially for being the definition of a 'WoW-Clone'.