This is going to be part from some posts I wrote on /v/ on the issue but here goes...
A huge part of SimCity 4's problems is just lack of testing and release before getting ready. Many features were scrapped, including having "snow" and "decay" masks for abandoned buildings (
link) as well as things like buildings that could expand over time (
link), then couple that with lack of forward-thinking technology (like being able to take advantage of multi-core processors, as SimCity 4 ran very slow on computers circa 2003). The Rush Hour expansion pack was probably just things that were taken out of the main game to begin with, and the flagship U-Drive-It feature was hobbled by the fact that it was all still isometric (and buildings had to render), and vehicle speed was tied to the network, meaning that you could have thrilling police chases on the streets...at 30 mph, which was what avenues and roads were set with (to be fair, that's the posted speed limit on major thoroughfares in big cities, but if it's not congested, people will usually go at 40 average...trust me, I know).
But Cities Skylines isn't nearly the thing SimCity 4 was. From the start, it's incredibly ugly. SimCity 4 was born out of the "real is brown" era of video games but it has its own color scheme (subtle pink, a layer of grime) that works well and provides a semi-convincing, if somewhat stylized, "real city" appearance (well, as "real" as 2003 technology got), whereas Cities Skyline has plasticky crap that looks awful and runs even worse. The core Cities Skylines game is incredibly poorly coded, and that's without mods. Many of the "good" Cities Skylines "art" pictures you see are made from games that have so many mods that the game chugs along on frame rates you could count with one hand. Moreover, the core game is awful. In SimCity 4's base game, there'll be traffic issues that shouldn't be there because the traffic simulator isn't well-built (based on "steps" rather than distance, or something like that--I've been sick with for nearly a week and don't want to go into a mini-explanation of how the traffic simulation works), but with Cities Skylines, the traffic simulation is even worse, with even JavaScript-based traffic simulations being more realistic (for example, congestion around a busy on-ramp should happen BEFORE the on-ramp, not AFTER, as cars have to slow down because there's so many cars trying to merge). My first and last time playing Cities Skylines ended because I had a small town (like literally a few blocks) and everyone kept sick because the hospital (again, looking something out of a Playmobil set) wasn't within a block of them, and then DYING, causing hearses to jam the streets. Meanwhile, the coal power plant wasn't even enough to supply everyone. Mods can, of course, readjust the numbers to not completely suck, but you know it's a bad game when the simplest thing has to be outsourced to mods. I haven't bothered with the DLC because I refuse to pay the developers for more broken concepts when they can't even fix the core game.
Trying to recreate an IRL city is going to be even harder, because of a few factors, zoning is based to roads (rather than just planting big subdivisions of residential, you have to start at roads and just branch outwards, and feels incredibly sticky), and the scale is off. With SimCity 4, each tile was 16 meters squared, which let you build semi-realistic roads and subdivisions (the isometric scale required making the building height a bit distorted to look more in scale, but was a good reflection of reality. Houses often were 2x1 tiles, which is about the size of single family home plats in America, and 48m^2 buildings could develop on zoned tiles (I don't know if 4x4 buildings could develop on zoned tiles, but that's about the size of "downtown" buildings), but in Cities Skylines, the tile size is smaller. This is great as it could allow for more diverse building sizes as well as some tiny hole-in-the-wall facades...but only theoretically, so the largest size a building can be is 32x32 meters squared (for comparison, that's about the lot size of a small gas station). Modders have made bigger buildings, but for a semi-functional city it means pencil-thin skyscrapers and housing for midgets.
Even Cities Skylines "best" features like free-form highways, aren't really that good. They all resemble Hot Wheels ramps and you can count the angles in them, and you'd need hours of fiddling with mods to make them even close to "right".
SimCity 4 also had a proper "scaling" system, known as "staged growth", meaning that even if you had high-density commercial, skyscrapers wouldn't grow overnight, there were 8 stages that your population would have to reach (and satisfying proper demand "caps", like an airport) before you could get the nicest and best skyscrapers (there's a mod for SimCity 4 that breaks it down even further), whereas for Cities Skylines, your city can go from nothing to a city of luxury skyscrapers (all pencil-thin, of course) in the in-game equivalent of a few months. SimCity 4 had a clear goal of where it wanted to go and what it wanted to be and featured polish (like an excellent top-tier soundtrack). Cities Skylines does not.
Part of the issue, I think, is that SimCity really was out of a different, bygone era, where PC games were segregated from console games with more complex, difficult-to-get-into games (advanced adventure games, wargames, RTS, simulations, etc.) that required thick manuals, which started to wither up under the multi-CD "FMV" games and finally died around 2000 when a series of events (mostly in the way that the industry moved) put an end to it.
While there hasn't been a good SimCity (or SimCity type) game since 4, indie games have reached the stage where they can make a mid-range product that hits most of the same notes. I really liked Prison Architect (unfortunately sold to Paradox, which publishes Cities Skylines), for instance...
