Just quickly-
Working economy and supply/demand chain. Player 'crafting' isn't important to me at all (nor player created guilds) but I would like to be able to help out local economies in a realistic way and see them grow. Trade is nothing in the game if commodities aren't actually doing anything meaningful. The most competitive enterprises could be the ones where players really help out trade wise, they should do better than facilities left to AI control, which might stagnate after time. In this way, players could work together to increase the population and economic activity of Systems, hypothetically being able to take a newly discovered System and turning it into a major hub of trade and activity over time (and thereby increasing its attractiveness to various Powers vying for control). That would be your 'Dynamic Galaxy'.
Apart from that... Adventure, Exploration, Mystery.