There's also the important adjustment that traditional box software companies have to make when they publish an online world game - it really is a different business model, a different marketing challenge, a different thing altogether. You have to treat it like a service industry - like cable TV, or phone service. Ultimately, whoever is making this sort of game is making a virtual space for players to interact in. To a large extent, the game itself is superfluous. It's about the other people.
Community is absolutely critical.
If you don't have a community in your game, its growth will almost certainly be stunted. Different types of online games of course have different types of community, but fundamentally, they all offer SOME degree of community formation, because they can't help it. As soon as you put people together in a shared space with some goals in common, they will start to form social bonds. Where they differ is in the character of the communities, in the tools and facilities that they provide to the players for community-building. If the game doesn't provide enough facilities for community, then the community will move outside the game and develop on websites, mailing lists, in IRC channels, or whatever.