Yeah, what differentiates a good game over a great game is whether it can survive when it loses a percentage of it's playerbase when a new game is released. If it can survive and those players return to the game when the 'Gee Wiz, look at this new game' phase is over, it can be classed as a great game.
Don't know if ED is in that situation yet, I consider it a good game, but the next few months will tell me whether it is a great game or not.
It's all subjective, but for me ED's a keeper even if dev stops tomorrow. I don't play it exclusively but it's a permanent install, like UFO enemy unknown.