For me, what entices -- or at least should entice -- players to logon everyday is to manage your empire. That's all of what the game is about to me, and should be about. Now, when I say "manage" I am using this term liberally as it's any combinations of actions related to your nation. From resource gathering to combat, elements related to these mechanics, actions and themes is what should keep players coming back day after day while attracting new players in the process. Getting players to stay logged in longer is another component, as they quality of the gameplay should be measured as well as the quantity: I do agree with your inference that players who feel there are reasons to come back, and more often, is in the best interests of the game and nurturing a community around it.