I've always been a fan of Co-Op since my old amiga days & null modem cables. Then I got a pc, and along came Diablo, Co-Op on that with a friend was awesome... There was the Dooms & the Quakes which had multiplayer, but the focus was on PvP... sadly this trend seemed to continue for years where the only solution was usually to team up against bots in a PvP game.
A few old work mates & I try to get together at least once a year (going back over 15 years) for a GeekFest, sometimes this is the only time we see each other for that year - the best fun is always Co-Op because of the varying skill levels & you don't really want to kill your mate for the 100th time... (PvP really does generate animosity, or at the least, disinterest).
The last meet we had we played Fear Combat Co-Op with as many maps as I could find. It was so much fun we ONLY played it all night, we all agreed this was the best night we've ever had, it wasn't the game so much as the Co-Op experience...
I recently downloaded the "Left 4 Dead" demo, played it solo for about 15 mins, then realised what a gem this game would be Co-Op, so I grabbed my 2 daughters & loaded it on their PC's and we played for hours. Now my 2 daughters are NOT gamers & very rarely play games at all, but the experience was so good for them, they didn't want to stop playing.
I'm glad to see the come back of Co-Op, and the way it's implemented in Left 4 Dead is brilliant. (I'm also becomming a big fan of Synergy for HL2.) Game companies take note!
As a side note to this, there is an issue I'd like to bring up. That issue is the need to have multiple liscences/purchases of a game for multiplayer. I have no issue where multiple people are playing the game & they don't live together, but to use my previously mentioned "Left 4 Dead" experience, if I wish to play this with my 2 kids I'll need to buy 3 copies of the game. Now here in Oz it costs $100 per copy - so I'd be up for $300 (for 3 copies) so we can play it on my home lan. There is no way I can afford this, especially for the limited amount of time the kids (they're very busy with Uni & jobs) would use their copies.
So, apart from technical/piracy issues, their needs to be something done about this side of things. Years ago, I don't remember the game, (one of the RTS games?), it would allow 1 retail copy to allow (spawn?) 2-3 other versions for Co-Op play on a local lan, perfect for home lan casual use.