FPS developers should definately consider bringing in RPG-style co-op play to the FPS genre. Currently, co-op play is understandably not that profitable a feature to put into your game because there are no real rewards for the players to co-operate. It's kinda fun to re-explore the single player game with a friend, but you have infinite respawns, and you're generally killing stuff without any direction. Meanwhile, RPGs have a party system so that people share experience points, gold, and treasure. They can swap items (most FPSs don't have inventories at all nowadays), weapons, amulets. I think games like Deus Ex or System Shock 2 are great for co-op play (ss2 did have co-op), but most FPS co-op isn't worth the trouble. Another thing about RPGs is that death usually does punish you in some way. You lose experience points, gold, etc. RPGs also allow players to cooperate by using town portal features, so people can join the surviving member in play.... buff each other with various spells/aura/enhancements. FPS games just have two guns instead of one. You can never directly enhance the other guy.
Battlezone 2, in my opinion, is the greatest co-op game to date. Just 2 players can engage in a co-op game that requires A LOT of coordination, and victory is truly at stake for the entire team. You do NOT have infinite respawns, as the game is an FPS/RTS hybrid, forcing you to worry about resources, strategy and so forth. Whereas most FPS co-op games, there is no reason to coordinate or call out locations of enemy forces or where they are amassing for an assault...... BZ2 forced players to constantly communicate, so that attacks could be met and stopped before damage was done. In short, it was truly cooperative play against a relentless AI. Due to its strategy gameplay nature, there were MANY roles each player could fill. Player one would be the commander, building units. Players 2-5 could all fill various scouting, defense, offense roles, in a number of different unit types. There was a sense of heirarchy, which is very important for meaningful co-operative play. There needs to be direction, purpose, roles to fill..... something that naturally occurs in team-vs-team multiplay, but is never seen in FPS co-op games to date. (Since monsters just sit around waiting for YOU to come get them, the gameplay is nothing more than run-and-gun)
Even co-op games such as RPGs and System Shock 2 tend to have little reason to truly communicate with each other to succeed. In RPGs, the gameplay is so simplistic that you can usually just walk into a battle room and start whacking monsters with the other party members..... and that's all. No real cooperation.
Co-op play's real allure is that the AI *is* weak. That is the point. It's inferior, and destroying huge numbers of them gives people a sense of great accomplishment. This explains the huge success of console gaming... the AI is often simple and easy to kill. It's why people like seeing Legolas and Gimli count up the hundreds of Orcs they slay, and so forth. But mass killing without a purpose gets old...