From Gauntlet to Left 4 Dead - the history of co-op gaming

Unless a game is specifically designed with an eye towards co-op play, it's based on very different rules. Death can't be permanent, but has to have respawn built into it. The game needs to decide if you lose things like keycards, or if one player getting them means that everyone can now open those doors.

Traps have to be designed with a view towards the player coming back after they've been sprung, instead of either getting through/time-travelling to try them again, thanks to the Quickload button. Doom, which let you play through the whole campaign with up to four friends, had it relatively easy. Run round the maze, shoot monsters, move levels.