This point is something I have really struggled to get my head around. I would just love to know how in Left4Dead they account for four people being in a team, all with their own stress levels. I developed a single player game, a battle against an AI. This allowed me to adapt the AI itself, making it smarter or dumber, faster or slower, luckier or unluckier as required. It’s easy to see how and what can be adapted in a single player game, but in multiplayer the issues are vast: differing emotional states, differing play motivations, differing skill levels, and differing styles of play.  Player motivation determines whether a player wants to rush through a game to get to the boss fights, or the good loot, or whether they are more leisurely in their progress, values exploring the world, reading quest logs, studying loot etc. Skill levels obviously changes how quickly a player gets bored or stressed out of their mind, how do you deal with that in a multiplayer setting? Styles of play is something ahoodedfigure talked about at my GiantBomb blog, whereby players can be very destructive, and all they try to do is break the game, whereas others just play it within the frames of what’s intended. 

I truly don’t know how to account for all these things when attempting to adapt to the player’s emotions. Doing an average could nullify the emotional states altogether, keeping nobody happy. Giving one priority over another will make that player happy and the other frustrated with the game. It’s also important whether the game is coop or a pvp game. In a coop it’s easier as the players should (in theory) all have the same goal, to progress together. But then again, some people are all about the me, me, me, and others are quite the opposite. In a PvP situation you can’t adapt the fight one way or the other, as that would make the more skilled player feel hard done by, and the fight would not really be fair. 
 
Man, I find this bit tricky! Any ideas floating about out there? 

Should really ask Valve about this... :P