Okay, I was exaggerating a bit, but basically:
Civ3 is getting more "Quirky" and "Humorous" graphics- basically the reverse of the transition from Civ to Civ2 or Civ2 to SMAC, which looked dead unexciting.
The number of civs is cut down to the original Civ1 numbers, in order to make them more distinct.
New models are taken in because "they found that people did a lot in that area" or "they were fun to play with" or "it added strategic depth", not because of realism.
And so on. The fun factor is the dominant theme.