I wondered too what it actually means.
Honestly i hope they don't go the MMO route. They have WoW already, comparisons would be just automatic and community would just split into the ones who like it and the ones who hate it. One thing i would absolutely not like is to have a world/maps full of random people and slow-ish MMO combat; Diablo is absolutely not about that.
Anyway, some MMO stuff may be pretty interesting:
- instead of small zones/maps, have a huge persistent world with bigger zones and no loading screens (waypoints still good)
- more character customization: both cosemtic (model) and though secondary skills like professions
- tied to above, a more robust crafting system
- while zones should be istanced for solo/small groups, you can have a central hub (or more) where you can see/trade/chat with other players
- static dungeons to tackle in solo/small groups; they could add lots of them over time to have more content/rewards to hunt for
- a gear grind that makes more sense than the one we have in D3 (way too fast, sets defining everything and basically removing nearly all the item hunt)
- while i don't like it, that is the chance to make trading right
Basically a fully online ARPG with some MMO structure. Stuff like raids should be avoided like a plague.