Emergence is a weird concept. I've played a lot of Minecraft and I wouldn't consider it to have a whole lot of emergence. It's certainly open-ended, but once you learn all the gameplay mechanics, the game gets a bit flat. You have to keep yourself interested in it somehow via community or huge building projects or whatever.

It's been awhile since I played Dwarf Fortress, maybe it's changed a lot since then, but I didn't find a lot of emergence there either. Once you get survival down, you have to make your own challenges. That to me is the exact opposite of emergent gameplay. Dwarf Fortress is the ultimate fantasy simulator, but as a game it's quite flat. A game should be able to hold your interest using nothing more than its mechanics if you want to say it has emergence.

The classic example of emergent behavior is Conway's game of life. The mechanics you see when you start playing the game, are exactly the same mechanics that keeps you playing the game years later. There's a depth to the flatness. But it's not a game. It doesn't present you with hurdles to overcome and a path to winning. It's emergent, but it's not emergent gameplay.

The other example is Nethack. There emergence seems to arise from the sheer combinatorial magnitude of the interactions that are possible. Kill a cockatrice and swing it's body around as a weapon, stoning all in one hit. Make sure you're wearing gloves. Don't descend stairs while you're holding a cockatrice and are burdened, or you'll fall down and the corpse will land on you.

There's also Angband. Here strategy and tactics plays the largest role. You spend a lot of time dying learning different ways of handling the games many, many, many ways of killing you. You learn the finer points of when it's okay to use a teleportation staff or when that's too dangerous and you have to use a scroll of teleport level. The actual difference between the two escape devices is very fine, but having one or the other, and the knowledge and experience to know which to use, provides a hurdle any budding player will have to jump before they can become a good player. There are dozens of these hurdles to learn, all evolved over decades of development.

Roguelikes have been the traditional bastion of emergent gameplay, with simulators and world-building games providing a different kind of combinatorial experience, another sort of emergent gameplay arises in fighting games. Huge depth of gameplay dynamics arise from little invisible boxes moving across the screen. Changing the amount of time an attack lasts by a single frame can make a huge difference in how that character fares against certain opponents.

It appears to me that each form of game, be it the top-down roguelike, RTS, fighting, FPS, has to find its own way to be interesting. MMOs come in many shapes and sizes, people play them for different reasons. I think the ultimate reason why you don't find emergence in MMOs is because combinatorial effects don't work as well when you're compounding human intelligence rather than algorithms. You can strike a careful, delicate balance game design-wise when all you have is one source of intelligence to contend with as opposed to thousands.