a point i have really tried to make recently on game forums is that in order for a game to be really successful longterm it must sustain interest. there is only one way to do that with the human brain (especially the brains of gamers) and that is through variety of activity
i'll use Defiance as an example again. i simply loved the core gameplay of Defiance. i enjoyed the missions, arkfalls, the run'n'gun style set in a story based mmo was really well done. but that's all it had.
a friend of mine, and hardcore longtime gamer, once said to me about a particular mmo that it wouldn't survive because there was no economy. i didn't believe him at first, but i realized he was right. he wasn't right that economy was the answer, but he was right in that the economy was fed by other endeavors, ie., a variety of activities. a variety of activities invites players to spend more of their entertainment time invested in that particular game.
now that variety could conceivably be composed of any number of things, but i think over time we have found that economies are one good way to do that when they are fed by player activities in the world. and back to Defiance this was an aspect that the game doesn't have. it should have surprised me, but it didn't.
the reason it didn't is the same reason i still have some hope for Defiance. and this reason is that i had preordered and played beta in Rift. Trion had set up a small grouping of good activities in that world. but after a week in beta i realized that this wasn't going to be enough to sustain interest beyond the first month or two.