Depends... there is really a fundamental difference between
a MMCOAG (massivly multiplayer co-op aquisition game) like WoW and a "Sandbox" like DFO. UO was the first "sandbox"
and EQ generally the first MMCOAG.

EQ did better than UO, for a variety of reasons and became the most copied model. But even though we all call them MMO,
they are really so fundamentally different in what they try to achieve that we should really start thinking of them as different genres.

My updated Lexicon:

MMCOAG : massivly multiplayer co-op aquisition game

Games in this genre generally focus around large, complex challenges meant to be tackled with a diverse group.
This genre offers a unified, persistant game world, and typically continually adds new challenges. This game can have PvP, but the PvP usually operates within the same overall context as the rest of the game: as a large group activity , basically just another type of challenge - but for this challenge, player groups compete in a structured fashion, generally for some type of "rewards"

Games in this genre: WoW, Warhammer, DAOC, EQ, L2


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PLAAG : Persistantly Linked Area/Arena Game

A PLAAG game is a game where your player/account exists in the context of a much larger community, however actual play
exists in an "area" or an "arena". Highly structured, the game revolves around pre-determined challenges, but operates in a completely "instanced" environment. The difference between a PLAAG and a MMCOAG boils down mostly to the intended "hook", that is: the main focus of the game - a PLAAG will typically focus on the main mechanic "usually combat" while a MMCOAG focuses primarily on continual character advancement.

It is interesting to note, that while Guildwars is the most
thought-of PLAAG , newer FPS games such as COD4, BF2 and others also arguably qualify for this genre - as experience is gained, new items are unlocked for XP/completing missions
and or challenges.

Games : Guildwars , NWN Persistant communities, COD4, Planetside

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Persistant Sandbox, or just "Sandbox" : A Sandbox is designed to provide an environment - as opposed to a structured content path, or a combat arena. The game is defined by its lack of definition, and exists , in its purest form as a "world simulation". UO was the first and best known, and at least at its inception even attempted to include complex ecosystems regulating things like reagent drops and mobile spawns.

The "hook" of a Sandbox game, is more or less that of having an alternative ego. Given real choice, its possible to create a real "character" and roleplay it as an idependant entity. It is also possible to engage in much deeper gameplay mechanics.

Sandbox are the most "social" of the games, because other users actions can directly affect the rest of the user base.
Sandbox games, usually exist as this social interaction being the main "hook" and focus of development.

Games: UO, EVE, Second Life