In my opinion, Civ1 was fundamentally simpler than any other Civ game. It is like the difference between playing DOOM and Halo. Civ 1 has very few units, very few civ types, very few anything really. That means that it is easy to keep the whole game in your head at once. For me, its a totally different experience.

Honestly it feels to me that Civ1 - Civ2 is the most direct upgrade in the series. Civ 2 was mostly just a better civ 1. From civ4 onwards, the series was a lot more willing to shake things up in its gameplay.