Cirom
aka J-town/NeonJ
I once found this coop server, back in 2.0, and it was the most fun I ever had in coop.
They had this system, where people would split into pairs. Each pair got to choose their own color, and those two worked together to finish the level. Me and my partner found that we were cooperating with everyone else as well, fighting of Eggman's bots together.
*Sigh*, I miss that server.
This is what I would like to see in coop, but I wish I knew how to convince people to play like that in my servers.
Another thing that I see that ruins coop is changing skin. Can't get past somewhere with Sonic? Switch to Tails and go over. You're too slow? Switch to Sonic. There's no cooperativity in that. If only there was a server where a Tails would actually help and lift a Sonic. If only I could host it.