Give us a LOBBY where we can creat games, name games for specific purposes, and choose what games to join, because this instant random mix and match !@#$ has got to go.
I hate to sound angry and demanding, but lobbys have been a fundamental aspect of multiplayer for as long as i can remember and i believe diablo 3 already has the facility for this system, i am primarilly thinking of the party screen when you leave game and your hero is standing there with 3 others, the chat box in in lower left corner allowing conversations about games is already there and you can add a games tab similar to the friends list tab that will bring up a list of all the games currently available for you to join.
I know Blizzard has the ability and the skill to make this happen, all they need is the will to do it, don't wait to implement this stuff with the expansion or some other dodgy corporate shenanigans for profit, the profit will be there regardless.
increase the party size to at least 6 as well please if you are so focused on co-op gameplay, let us co-op with all the classes in one game.