Jay Wilson, and other folks from Blizzard have been promoting Diablo III as a coop-centric game. A great deal of development time has been working towards this end. I like this idea, but I think Blizzard could have done a much better job at implementing robust tools to facilitate this type of play.
Currently I am envisioning a Diablo 3 where you enter into an inferno game and begin working together with your teammates. Before venturing out to kill demons and starting to build up Nephalem Valor charges, you must set up your skills, since changing your skills mid-game will remove the buff. Who you are playing with and what skills they are using will determine what skills you are going to use. If your teammates are already using abilities that snare enemies you might be inclined to use abilites that increase your teammate's damage since getting another snare ability would make your group too defensive without any strong offensive abilities.
This brings up a lot of questions. Are these options going to be discussed through typing messages in chat? One can image how cumbersome this would be. No voice communication closes so many doors towards cooperative play. You can't call out for help, or request a heal from another player very easily. And you cant discuss skill planning strategy prior to venturing out of town.
Outlets for the average player to form connections with other players are there, but very limited and/or inconvenient to use. Things like Guild Support, and Voice Communication would go a long way to helping players establish communities, partnerships, and friendships in-game. This would make Diablo III a truly "coop-centric" game.
Update: Shortly after writing this article, Blizzard announced a Clan system to be released for Battle.net at the same time as StarCraft 2: Heart of the Swarm, which will be later this year. This is good news for the Diablo III community. Even if it will come months after release. =)