Cities: Skylines is a very good video game, but in my review, I noted it had a few problems. I also noted, however, that most of those could probably be fixed with mods, and that's exactly what's already happening.
And no, I'm not talking about road porn. Or buildings (I'll cover buildings and maps very soon). I'm talking about actual changes to the way the game's systems operate, things that can not only alter the way a city works (to bend it more to your liking), but can just improve your overall experience.
You should subscribe to all of these mods in Steam's Workshop, immediately. They'll make the game better (provided they work 100% of the time…some of them don't, so you might want to mess around first).