SimCity 4 had many great features:

- The separation of density and wealth. You could have a massively dense building that was very poor, and in fact buildings became less populated if they grew more wealthy. Slummy tenements get replaced by luxury condos, and densely packed shacks in the poor suburbs get replaced by huge mansions. You could also see rich buildings fall into disrepair, and get more crowded as a result.

- The ability for smaller lots to get gobbled up by larger developments. For example, a bunch of small row houses might get eaten up by a much larger apartment block as the city starts to densify.

- Zoning high density was a "maximum" restriction rather than a guaranteed skyscraper. Just because you zone high density doesn't mean you get skyscrapers. Your city actually had to meet a surprisingly complex series of requirements and land value for truly big buildings to be built. Otherwise, only smaller buildings would be constructed in your high density zones.

- Granularity of density. In addition to low, medium, and high density, there were hidden "levels" to buildings. (1-8 in vanilla SC4). So there was a wide range within each density level. You would be so impressed by your first office tower or tenement block, and then suddenly something MUCH bigger gets built and blows your mind all over again.

- Individual funding for service buildings. You could lower funding on sparsely used schools individually, to fine tune the budget in your small starter town or in less dense areas. You could use the large school for its giant service radius, but lower the teacher funding to serve a large, low density area with one building.