Watching the Civ4 "making of" presentation was quite illuminating.

Some things that caught my attention:
1. Having a working build already in the very early stages of development allowed for quick iterations and early playtesting. I'm curious whether Firaxis was able to maintain this approach for Civ5 (because for Civ5 the game engine had to be developed in-house and in parallel, whereas Civ4 used the existing Gamebryo engine). If the Civ5 devs had very little opportunity to see their design in motion during development, then this might explain why the gameplay in Civ5 feels less polished.
2. "The units are the most important thing on the map, not the terrain, the terrain shouldn't 'hug' the player's attention". Also, interesting to hear that they scrapped a whole system of realistic looking terrain in the middle of development because the players couldn't easily see which tile was what and therefore had less fun. As Soren says:
Since I had a _lot_ of problems with both things in Civ5 (terrain tiles bleeding into each other, and units being hardly recognizable on the terrain), I wish they had continued to follow this rule instead of concentrating on "organic looks". This may be a preference issue, but Civ4 clearly matched mine better in this case.