So I was up quite late last night.
I was pondering what makes the game so compelling for me. Of course it has a lot of the, hmm, let's call it 'opportunistic synergy optimization' gameplay that I like (see also Mythos), and when you chain things together properly you can build nifty victory engines, so there's also some good conditional logistics.
The flavor is also very good. It's very evocative without being overly detailed, and the thing I noticed last night is that when you get a decent strategy going, the cards fit together in a way that builds a little story.
But I think the thing that keeps me coming back is the excellent timing of the win condition. It feels like the game generally ends just when you're getting everything into position, so the winner doesn't usually have time to pull a long way ahead of everyone else, instead the finish line suddenly comes into view and you have to sprint for it. Which means that you're always left wanting just a little bit more. "Just two more rounds and I could have done X and Y!" is a common post-game comment. It's a very satisfying experience, but often not a satiating one.