In D2 it was very rare that people would play actually cooperatively. In the early days of D2 it was all about playing a MP game and soloing while everyone else soloed too. In general, even in games where people are doing the same thing (ex, cowing, bloody runs, baal runs) it's more about everyone doing their own thing while everyone else does the same.
IIRC they wanted to make it more focused on actual team play, which is much easier to balance with smaller team numbers.
I am betting that games wont quite be the same in d3 in that each instance will be it's own self contained game, so there will generally be much less need for having all your friends in the game.
Just think about the average D2 game though, It's a baalrun, you have 1 person teleports to the throne room, everyone goes in there. 2 or 3 people do all the killing (hammerdins probably.) and if it's full 4-5 people are just leeching experience. Then everyone smacks around Baal for a few minutes and repeat. There's actually very little player interaction other than trying to get the drops before the other guy, and I think they're taking an entirely new hollistic look at it.
In all we don't really know how player interaction and setup will be aside from the lower player count instances, because so little of the game is known. We know there will be some sort of "overworld" but aside from that we really don't know how the player interaction will be geared around, how trading will happen etc.
