It looks like you're new here. If you want to get involved, click one of these buttons!
Mods please pardon the James Carville reference, it's not directed at anyone.
For the longest time people have attributed WoW's success on marketing and catering to casuals. With the release of MoP and with so many big games that came out prior that just didn't have staying power, I think the pattern has become much more clear.
It's the content. The amount of content more specifically. WoW doesn't cater to casuals, though it's bar to entry is quite low, that bar has great range for it's highest level of gameplay (many will argue, but that's not this thread). WoW caters to content locusts. When the content locust article came out and pointed to this group of people who run to every new game, hype it, and consume it within days or weeks, it showcased how new games seemed like they didn't have enough content.
But if we really look at WoW, both currently and historically, it's been about longevity of content. Oftentimes there are artificial limiters (like dailies) but Blizzard found a way to cater to the locusts way back in 2004 (probably without knowing it at that time, but has since perfected it).
Content locusts not only consume as quickly as they can, they are determined to consume everything (or most of it). So if you have raid content, they want to finish it. But not just finish it, they also want to do it on hardmode. But then they also want the achievements. But then they also want to gear the legendaries. But then they also want to get rare mounts and pets. And so on.
Blizzard has stuffed WoW with so many different types of content, that the locusts can't just show up, finish everything in 2 weeks and then go on to the next. Much of it may be easy to complete (hence the idea that WoW caters to casuals), but there's just so much of it so as to appease the locusts.
Basically, WoW answers the questions, "So what's next?" In so many other games, people run to the forums and say, "I'm max level, I've done this and this, what else is there to do? So and so failed." But in WoW, there is almost no end to the available content, which is why there is more burnout. I know, on these forums especially, that many here simply don't like WoW's questing or combat style or some combination of things.
MoP is very close to Burning Crusade level change. WotLK and Cata were simply refinements (or for some, dumbing down) - but BC was essentially modern WoW while Vanilla was Everquest+. BC brought arenas, "welfare" pvp epics, badge gear, heroics. It is one of the most highly praised expansions and most people's favorite.
MoP, while not as drastic, is functioning in a similar manner - again creating more content for the ever hungry locusts. Pet battles, both pve and pvp are basically an entire game in and of itself. It has much more depth than people are giving it credit for and you will see that people are going to take it quite seriously, and the rarer pets spawn at certain times of day or year or under specific conditions - that's awesome.
They revamped the talent and glyph system and some classes have completely new resource management. For any casual player, WoW has more entertainment value than any other game, you can play it forever and still have more content to do. But that's why it's also a hit for the locusts, they just can't consume it all in a time frame that would make it look like it's failling.
You really see drops in WoW population after certain mile markers, 3 months, 1 year, 2 years, few months before next expansion. Other games have been showing huge drop offs in 2-4 weeks. Again because people can chew through content like crazy. Now, none of this means that WoW isn't slowly fading, but some of the things which people generally despise (raids, dailies, grinding) are actually what create longevity of content. With nothing to work towards, people just leave.Da locusts, gotta feed em.