There's a philosophical difference in definitions here. You can force group play to get anything done or group play can be optional. Both ideas fit quite comfortably under the definition of an MMO which is quite simply a Massive Multiplayer game that is Online. People make obvious attempts to stretch that definition to fit their own biases, sometimes dishonestly but it's an irrelevancy. Diablo III is more MMO than not. It's mostly that the emphasis is not necessarily on grouping up. But you can play with friends or clanmates.
It's possible to have this same exact experience in retail. In fact it happens all the time in retail at end game.Yesterday i was leveling in Westfall. And i meet this other dwarf priest. He was killing harvests and becouse i was doing same q i invited him. We end up playing together for abother 45 minutes, finish several quests what would otherwise be pain to do solo and actualy talk to each other. Then he left becouse he had to go to the hairdresser. I have had this type of experience in moder game since WOTLK. And exactly this type of experiences is what make classic better mmo than retail.
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Clearly there are no "subscriptions" in Diablo so your comment doesn't make any sense whatsoever.