I've been focusing on Civ3 for the last several months. My computer which could play Civ4 died, and it took awhile to save up for a replacement. (The new one, BTW, should be capable of playing Civ5, if I end up giving it a try.)
Like many of the folks here, I like big empires -- but not huge empires. More specifically, I like empires of approx. 40-60 cities, where 30 or so actually need my attention. A sprawling empire of 150-200 cities, where more than 100 are just sitting there, making Wealth, not growing, is not particularly fun. I like building stuff, and conducting medium-scale wars.
Thing is, I can do that in Civ4, too. Trying to win with 2, 4, 6 uber-cities is definitely not fun. But I can run a 40-city empire in Civ4; I can develop the resources, decide whether to cottage or farm, track my great people, and influence my neighbors on the sly with missionaries. Bottom line: in Civ4 there's more stuff to build, and I'm a builder at heart.
I agree that the graphics are a bit goofy. I chuckle and shake my head when I zoom into a city, and see the King-Kong-sized archers standing astride the buildings in defense. I think the suicide catapults are a weird game mechanic, but I understand it and use it. I like the rock-paper-scissors sort of strengths and weaknesses of certain units, and thinking about promotions. Again, it's like building a unit -- by promoting it, and by upgrading it.
It's true that Civ4 has some more things you can do. More worker function, Civics instead of governments, the religion and the health thing can be used for diplomacy etc... but this also sets me off. You can do all this with what 4, 5 cities and that's it. It's even that when you build more cities, the others become much weaker.
In Civ3, when you have a strong core, you build on, corruption? Build Forbidden Palace, move on. Build/concquer another core of cities. Corruption? Go commie and build Secret Police HQ... Or make the cities moneyfactories.
In civ4 when you decide to make a happy no-war supercountry... and one little war arises, you're doomed. If you're a warmonger, those big supercities are almost impossible.
Don't like it.