I think one of the better improvement in Civ3 is the AI. It's quite challenging right now and I'm very pleased.
My 'gripe' now though is that not all games I'd played Civ1 & Civ2 was for challenge. Sometimes I'm just looking to kick some b@tt and/or have a quick game.
Unfortunately, this now seems impossible in Civ3. Even on Chieftain level the comp. is very challenging and games take quite a long time.
Maybe I'm just not that good yet, who knows, but I think from the evidence I've seen I'm right about this. Can't please them all I guess...