Really, I haven't the slightest.
So what does it take? Even for the best of us, we all have something. A switch, a weakness, something that will make us stop and stare like some animals when they look at shiny objects. People can play GTA and get lost not even doing missions, but just driving around. Or in a game like Oblivion or Fallout where you can just pick a direction and walk until you find something. Driving games have the technical feel of real world cars, or just the fun of driving one faster than is actually possible (forgive me, I'm not a racing or sports game player). RPGs can be just as much about watching a story unfold as it can be about proving your dominance over the gameworld by leveling up (usually) pointlessly. MMO's, in general, trade on the basis of dropping players into a large persistant world and then telling them to "figure it out."
Maybe that's what it is. The ability to explore, but being able to do something on the way.
As much as I played them, I could never get into the GTA games. They always talk about all the things you can do when really, outside of missions, there actually ISN'T anything to do except drive around and purposely start shit. Yeah, you can explore, but you're limited to a car and/or a painfully slow running speed. There is very limited purpose to taking the time to explore the world, which is why the creators have to put in so much peripheral content. Without it, the majority of the open world they created is suddenly without purpose.
But then , Oblivion held my attention (until my friend created the 4th or 5th new character to play the first five Dark Brotherhood missions for the 4th or 5th time, on top of killing my first XBox in the process... *grumbles*), as does Fallout3. Like GTA, large open worlds, but the worlds have purposes, the peripheral content doesn't seem like it's there simply to pad out everything. That, and the worlds don't feel generic and lifeless, something I've felt despite how much Rockstar has tried to make the world more lifelike.
And then you get to games like Crackdown and Prototype (the game which caused said 7 hours of playing). Open worlds with very little actual content beyond the main story missions and a small amount of peripheral missions. Yet, they manage to hold the interest of people. Why? One word: locomotion. The world is a playground, and the developers made it fun to play in because they give you so many ways to get around, and so many reasons to explore. Like in many games, your character gets stronger, an in these game it's tied directly to exploring the world outside the confines of the mission structure. Now, simply going around looking for these upgrades is not terribly interesting, but it can get interesting when getting to them involves superspeed, flying leaps, running up buildings, and causing wanton destruction (on enemies at least) with high explosives.
And that, to me at least, is more fun than driving around in a car and running down a street where nothing happens unless you purposely go out of your way to cause shit. Sometimes shit should just happen, and then can you get involved. Or not.