Diablo 2 actually had all business logic in the client itself because it needed to support single player and LAN play. They were able to just use the BNetD project to run Diablo 2 compatible battle.net servers with very little D2 specific logic.

A better comparison is probably WoW. Like Diablo 3, all of the business logic is server side and it did take awhile for emulators to pop up.

Mooege is out already. Not sure how good it is, but it seems to be out for over 6 months now (started before open beta).

AFAIK, it doesn't do anything. You can use it to connect to a server and spawn monsters that stand still.