My problem with Civ3 was that it just felt...burdensome. Hundreds of units to move, dozens of cities, bazillions of buildings and improvements to be built in every city, and so and so forth. I just got frustrated with it all much quicker than I ever did with Civ2 for some reason.
Exactly how I felt. And that's weird, because I used to play Civilization I for hours and hours and hours. Maybe it was because I was younger then. And had more time on my hands.
Anyway, I am very tempted to pick up Civ IV. Still on the fence.