ahhh after thinking a little bit more, i know the answer now,
back then the battle.net was and i think still is the best platform for multiplayer meeting with friends and communication with clans and stuff. battle.net should on my oppinion have all multiplayer games.
you can whisper with your friends, when they are online, you see when they are online, its brilliant.
and the other great feature was that everybody, no matter if he has a good pc or bad, could open a server for free, based on his computer.
jeah and the chat system like irc kind of. i really liked that alot, so i never had the feeling i am alone playing diablo2.
the chat was ideal for trading items and making/opening new games or runs. ahhh good old times.