The formula for the perfect game may be an area of hot discussion, but most would argue that games that are easy to get into and difficult to put down goes along the right lines. Players' early experience of a game is critical: this is the period in which they're making up their minds whether to devote more time, and often money, to the rest of it. 

So how do we make games with a polished early experience? Let's take some lessons from a game which I think does it incredibly well, PopCap's Plants Vs Zombies. 

MMOs are games where you play combat primarily in the user interface rather than the game world.
I think this is best realised in the classic ‘standing in the fire’ error of new raiders: essentially people stand in the fire because it is an element of playing in the game world, where levelling-up has trained those players to instead play in the interface. Combat is in the cooldowns; you watch timers, health bars, debuff bars, and only when you get to raiding or the more ambitious small group dungeons do you need to start looking into the game world too, in order to step out of the fire, dodge the laser beam, jump over the furious shrew of ruin.

Presentation about interaction design with a focus on game mechanics