Luckily you could separate slightly further than this
I don’t think that it is good enough to create a world and just allow more than one player to run around it at once. To be truly co-op you have to have some sort of gaming mechanic that actually encourages players to work in co-op, be that in specific co-op mechanic like Army of Two: The 40th Day, or a system that encourages co-op play e.g. Left 4 Dead. Left is a brilliant example of a game that does not force players to work alone or in co-op, it just feels natural as the game is programmed so that harmony breeds success. LEGO Harry Potter was fun to play together, but I found that splitting up and smashing things separately bred success as you got more done in less time. In previous LEGO games it was annoying to be stuck in the same screen, but at least you witnessed all the hidden Easter Eggs in the game. In LEGO Harry Potter my partner and I often found ourselves revealing hidden secrets whilst the other player was occupied elsewhere, a shame.
One aspect that brings players closer together is death, not in a Twilight Saga hand holding type of way, but in the mechanics of the game itself. Crackdown 2 does offer a revival mechanic, but it’s not necessary to succeed in the game. You can respawn and make your way back to your teammates with relatively little impact on the gaming experience. On the other hand games like Gears of War 2 require that both players are kept alive if you want to finish the game. This means that you always remain within a safe distance of one another just in case you need to be revived.
Are open world games with co-op aspects an attempt at pleasing both single player fans and co-op fans? In a more linear game like Halo 3 you can experience the same set pieces as one group, but with massive playing areas you can often end up playing a separate game on the same map. Throwing players into a huge world with no co-op specific challenge will make traditionally single player games more fun, but is it enough? I don’t want to see a world were you are tied to a co-op partner on the same screen Fable 2 or Clash of the Titans style, but I would like elements of the experience only to work if you are together. There is room for all types of co-op game be it single screen Zombie wrangling or open world orb hunting, but for me the best experiences are always those that you have characters side by side on screen, no matter where the other person is in the world.