What is an MMO? I think when we talk about this particular topic the definition itself becomes personal and very subjective. At a basic level, the wording must mean something though. Massively. Multiplayer. Online.
Massive doesn’t have an objective definition and is completely relative. What one person thinks is big could be small to another. At the highest possible level, we can infer than massive means big. So let’s say that Massive, in this case, is when you compare to the typical games. A typical game has enough content to last 12-20 hours. A massive game would require content that extends beyond that time. While you could play Solitaire for years (who hasn’t), you wouldn’t call that massive. The content needs to provide diversity. Counter Strike might have the same basic elements but the randomness of other players increases the content value.
Multiplayer is very objective. Either you can play with other people or you can’t. Pretty much every game on the market today has multiplayer. If the actions of one can affect the other, than it’s multiplayer. Simple enough.
Online is also objective. Either you need a network connection or you do not. If you don’t need to be online (or a LAN) then it’s not online. Co-op on a console is not online.
Using these terms we can come down to some agreement on what games should be considered MMOs, if their primary mechanics fit into the three criteria. EvE, Rift, WoW, LOTRO, DDO and the rest in that genre are certainly MMOs. Call of Duty has that primary focus, even if there’s a single player component. Minecraft is the same.
Some games do straddle the line though. Assassin’s Creed has MMO components but it isn’t primarily focused on those 3 attributes (certainly not the massive portion). Mass Effect, Uncharted don’t either. Diablo 3 might sink hundreds of hours but the content is the same throughout and the people around you don’t affect that. If I said that you needed to connect online to play Final Fantasy X, you could easily argue that it is much more massive than Diablo 3.
Looking into a crystal ball we can see some patterns emerge in gaming as a whole. The Multiplayer and Online portions are pretty much going to be the norm from this point forward. The kicker is in the Massive portion. Skyrim is massive. Batman is not. Most indie platforms are unable to be massive – unless they have procedurally generated content (Rogue-like games come to mind).
In the end though, what difference does it make if D3 is an MMO or not? Do all of a sudden all future MMOs have to follow that game? Is it easier to compare D3 to WoW or EvE to WoW? Do you even want to? Games today are more than simple statements. Games are experiences and experiences are personal.