I was a happy bunny in 2008, because it was the year when designing games specifically for co-op play finally entered mainstream thinking. Sure, games have had co-op modes for a while, but they’d usually been a bolt-on extra - at best you could add maybe one extra player who would be ‘along for the ride’; disposable, an afterthought. At worst, it might actually undermine the play experience, because the game wasn’t deliberately balanced for 2 players, or maybe you’d be limited to a small section of the content in co-op specific play modes.
I finally got chance to play Left 4 Dead co-op with my wife rather than single player with all AI teammates, and it’s an entirely different experience. I’d heard this of course, but even so I found it surprising just how much difference it makes. Having a real person yelling for help as they get jumped by a hunter after hanging back behind the others, running scared from a marauding tank together, or frantically trying to help a team-mate up so you can get back up into a defensive choke point before the wall of dead flesh that’s charging down the abandoned subway track crashes over you, it’s simply enormous fun.