Now to the latest of my cooperative gaming experience: Battlefield 1942 (and specially Desert Combat):

Desert combat is a great fun game. It does most everything that Operation Flashpoint made before, but in a simple straightforward arcadish style that can get people hooked really fast. (Operation Flashpoint had an uneasy learning curve for those that were not self-motivated). And it comes with a native cooperative mode that genuinely fits the game. (Add inserted here: Why are the helicopters so damn hard to fly? They were so much better in Operation Flashpoint)

Of course, getting the game to work on a LAN can be difficult with certain "versions" of the game.

But, if you do manage to get the right patch-upgrades-add-ons-modifications-levels combination, you are in for a treat of a game.
Cooperation is required and soon becomes very focused: each one finds the weapon-vehicule combination that works best for them, and then can easily see what need to be done in the map: which flags need capturing-defending or where to drive the offensive. (Real battlefields are probably not as neat in organizing the tasks and goals, but who cares?) Therefore we achieve something beyond cooperation: self-organizing or "autopoiesis" as Maturana would put it. This is very good.