It's definitely lacking the feature from SC4 where I can click on a building in traffic view and see the routes residents take from home to work. At least there's a mod that does that. And like people are saying, getting a city that runs nicely isn't really challenging- however, it's pretty difficult to alleviate traffic jams and solve problems once your city gets really large (say 100k people). I think we might do well to remember that most of the difficulty in SC4 was in realizing that you needed to decrease the budget of your service buildings as soon as you placed them- after that I felt like it was almost as easy as Skylines is now. Besides, SC4 now has an expansion pack to benefit from as well- I don't doubt Skylines will be improved with DLC later, plus mods are coming out at an alarming rate.
All-in-all, I feel like Skylines did to city builders what X-com Enemy Unknown did for tactics games. It's a bit more casual, but it's still fun and presents challenges (mostly traffic related but SC4 was the same in the late game). I don't miss the grind of having to build several cities with import and export demands before a really large city could be built and I
REALLY don't miss
demand caps. Yeah, remember those? How you couldn't get residential demand because you forgot to make farms just to build a farmer's market and immediately tear down the farms?
Aside from some demand for more UI and graph information on traffic, I really do with that industrial/office levelups based on services used the ACTUAL ability of your services to affect them (EG how fast police, firetrucks, and ambulances arrive) rather than arbitrarily needing to be close to services, especially in a game where the actual intricacies of traffic are simulated and very important. Oh, and I'd love to be able to place bridge supports in the middle of four-lane roads.
Anyway, the point is, Skylines isn't perfect, but Sim City 4 wasn't either and I think Skylines makes a great successor- besides, it's still got a lot of time for more elaborate mods and DLC to come out.