Everyone loves a good story. For many gamers, the story is what drew us in to RPGs. For others, the mechanics of the game hooked us; maybe it was the overall presentation. I think I can say with confidence, though, that the story has and always will be the crux of an RPG, especially in the area of MMOs.
Over the years, I've enjoyed engrossing myself in the lore of games. The story, to me, is the focal point, whether I'm following an arc or coming up with my own. The story gives me reason for doing what I do. That being said, I've taken the time to consider how critical the story in to an individual MMO and found that it revolves around several other factors.
The Focus of the MMO
Depending on what the game is designed for, the story may take a back seat in some MMOs, only being used to justify (sometimes weakly, at that) events or mechanics that appear within the game. Sometimes you don't need story to lure the player along. PvP games come to mind when I think of this. When all is said and done, you don't really need much justification to bash someone over the head with a mace aside from base desires (bragging rights, loot, etc.).
Although PvP is one element that can take the place of story as the focal point of the MMO, others can as well; even PvE (instanced raids, for example) can also detract from the story element if given the primary focus. Is this justified? Should others game elements be placed in the forefront to overshadow that upon which it is based? Personally, I say no.
The Story is King
I don't remember the developer who said this, but I'll paraphrase and say that the story is king when it comes to MMOs. The story and lore around it justifies the world in which we play. Everything else, I think, is secondary. Combat (PvE and PvP), crafting, exploration, NPCs, races and cultures, all hinge upon the solidity of the story.
Now, some of you may disagree with me for different reasons. I realize that others prefer more direct styles of game play, not caring or unwilling to endure lines of justifying text that fill out the context of the game world. To this I say that all other aspects of an MMO are important, especially to fulfill those short-term needs and desires you have when playing a game of this magnitude. The story though, if you haven't already guessed from my writing, holds a place that transcends game mechanics. It encompasses everything and fills in the spaces between game mechanics and gives them the fluffiness they need.
To give an analogy, think of a multi-layer cake. Each layer is solid and represents a polished aspect of the game. Combat, content, crafting, economy, exploration, housing, and many other layers comprise this cake, but these layers are just cake; there's no icing. The story of an MMO is like the icing. It's everywhere, filling in the space between the layers, containing different flavors according to where it is, and even giving that fancy filigree on top that makes you look at the entire thing and go, "Mmm, that's one good-looking cake."
Now that analogy isn't perfect, but I think it expresses how I feel about MMO story. Without the icing, you just have dry pieces of cake. To go one step further, you could say the icing holds the moisture inside the cake, keeping it fresh. I could go on and on, but you should have the idea.
Read Between the Layers
Some games out there have really cool features. Some have good PvP, others PvE, etc... After you engage in these nifty areas for a bit, the same content gets old. This is where story plays a second role just as important as the first. After laying down the groundwork and lore for the player to justify their role in the world, changes in the story give rise to the development of new content. It breaths new life into old areas, and gives the player the effect of being in a changing world.
This effect, in fact, is something I find as being a barrier that many MMO companies face. From what I have seen, most expansions simply add new areas and content that spreads out and away from the original world, leaving it to stagnate for the most part. If any readers could fill me in on MMOs that don't follow this trend, please let me know. The one obvious MMO breaking this mold is WoW with its Cataclysm expansion that claims will remake the old world. We'll see if the story matches up to the level of change it brings.