Diablo is not a persistent world. It only "exists" when people go there to open up instances. Swamp of no Hope was always up, even when empty. And there was only one Swamp of no Hope. Just having your shit stored on a cloud doesn't make it an MMO.
The key features for me that make something an MMORPG is the one world that every player is on, that you can run into players that you do not know at all. There's room for SOME instancing, but once you only see players in trade hubs like in PoE and never meet anyone in the world then it's just that, an online game with multiplayer options.
In EQ, you could run into a guy you didn't know, who did something completely different in that zone than you are currently doing. That is the key for me that defines the "massive". It's not the number of players on the server, it's the size of the open world where you can meet others. And a 100 square feet merchant hub isn't massive.
The Diablo reference was to having an online/offline world, not about the nature of the online world. The online world is persistent and always there for everyone. It's always the same from every person and what you do in it is universal for everyone logging on. The only difference is in the
gameyou can load up a single player version and play the game like Skyrim, but
that single player character, much like in Diablo, can
notgo into online mode. (Hence the diablo reference--you couldn't port your single player toons into the multiplayer game).
The online Multiplayer has everything you've said an MMORPG is supposed to have--which is why I'm confused as to why it's not considered an MMO (It's not the traditional MMO but it is an MMO). The multiplayer option is a persistent world that always has other people in it. The only difference is servers are dynamic, unlike current modern MMO's, so seperate "servers" can be merged or separated depending on population. But it's not like they are only together in town--he makes reference to seeing people in the wild, and helping them. The multiplayer game is essentially an MMO, just with WoW's new dynamic server system to make sure the population stays within a certain threshold.
This has a ton of benefits. It has some negatives too. Like say, I'm shopping at a store and I see someone talking to the air--this can happen with dynamic servers, because I may be synched with one person, but not the person they are talking to and I absolutely hate this aspect of it because it shatters immersion (Supposedly, his algrythims are super advanced though and after a few weeks this will never happen, because the game will know that person A and B hang out around C, so it will always load them together for C--I will have to see it to believe it). The benefits though can be numerous. You don't need things like dungeon finder, because everyone is already on one server--for example.
It's not an MMO in the traditional sense, but I'd say it's a new "kind" of MMO, for sure.