I've only begun playing it. The way I see it, there are neither missions nor factions, but you can make your subsequent playthroughs more difficult by enabling disasters or starting in harsher environments (worse climate, less space). No tech trees either; all the buildings are available from the very start. So the replayability lies mostly in the challenge. There's also talk of some modding tools coming soon, so there might be some worthwhile mods in a few montsh, although they can't add anything significantly different to the core game.

I agree, the price is a bit too steep, but I bought the game just because it's a great mix of some survival games and city builder games like the Anno series, which I dislike because tends to get bloated as your city grows, and I like managing smaller communities.