I imagine it's pretty easy to appeal to city building fans in this post-SimCity world. All you'd need to do is tell people how much you're not SimCity. Offline support? Sure, you'd say, rolling your eyes at games that didn't offer such a basic feature at launch. Big cities? Of course, you'd reassure, your tone making it clear that to do otherwise would be inconceivable. Day-one mod support? Absolutely, you'd half-smile, presenting a video you'd prepared on just that subject.
Coincidentally, the creators of Cities: Skylines have prepared this video on just that subject.
If you'd like a run-down of what exactly that modding support allows, you can see Colossal Order's guide here. So far, it's mostly asset and map editing, as well as a "presently minimalistic" modding API. According to the wiki page, "available features will evolve alongside user wishes in the future."
Cities: Skylines is due out 10 March.