It's difficult for me to answer with specificity, having never played an MMO, 2D or otherwise. The most obvious answer would be that AVWW isn't an MMO, and isn't patterned after them.
That said, I've played a variety of single player games that imitate MMOs, and I've seen brief bits of Maple Story played. Near as I can tell, MMOs like MS seem to be about social interactions, PvP play, and grinding up your levels. Conversely, AVWW can easily be played solo or co-op, doesn't emphasize PvP play at all (to the extent it is even possible), and has EXP gain and such structured in a way that you
can't grind monsters (they don't give EXP).
The focus of AVWW is more along the lines of traditional pre-MMO adventure games, mixed with a bit of city builder mixed with a bit of grand strategy, mixed with a bit of other stuff. You're shepherding your entire civilization along, improving it either by yourself or in tandem with other players on a server, and everything you do matters long-term to the game. Death is permanent for your characters and for NPCs, and so on. But this isn't a hardcore penalty -- you just get another character when you die, which you will -- it's instead a story-building device. You're writing the history of this whole civilization as you play.
There's more to it than that, but that's where all the queued-up reading you have comes in handy.
The game is, in almost all respects, closer to a roguelike than an MMO. Hope that helps!