I'm having an unanticipated problem with this game - I don't want to stop playing long enough to post about it. Even now, I'm driven to distraction, thinking about what I'm going to do in my next game. There are so many widgets and fiddly bits that there is almost always something that needs doing right this second now and balancing these competing opportunity costs with your long-term goals is an immensely satisfying intellectual exercise.
I love how the different pieces of Civilization IV fit together, exploring the map for exotic resources, researching technologies that allow me to exploit them, choosing the location of my cities based on the terrain and availability of resources (as opposed to earlier civ games where you wanted to put a city on all available land, so as not to waste any of it), determining whether you want a city to focus on food, gold, or production - there's something beautifully harmonious about the way things will suddenly click and your civilization's economy will become a powerful engine for your ambition.
Civilization IV also has a number of quality of life improvements over it's predecessors. The in-game clock is useful. The AI will not enter your borders without a free passage agreement (this was the second most annoying part of civ3 - the AI would move settlers through your territory, clearly intending to box you in with settlements on the far side of your empire, and then they would get annoyed when you told them to back off). It generally takes fewer clicks to navigate through the interface, and you can almost always consult the civilopedia.
But I think my favorite feature of Civ4, the one I missed most acutely in Civ3, was the "Quick" game speed. The increased building and research speed tends to mess things up if you play a military game (because your units are obsolete almost as soon as you get them to the front lines), but I, being a shameless peacemonger, tend to like it because it means I don't spend a hundred turns doing nothing but pressing "end turn." Granted, it does tend to rob the game of some of its epic scope, but that's what Fall From Heaven is for (seriously, I cannot overstate how much I'm looking forward to Beyond the Sword).
On the other hand, the advantage of quick mode is that in a mere four hours, I was able to complete two whole games. I lost the first one (on Settler difficulty no less) because I never bothered to expand to a second city and was just a few turns shy of finishing the spaceship, due in no small part to my unfamiliarity with vanilla Civ4's tech tree. The second game I bumped it up to Warlord difficulty and won handily, most likely because I didn't dick around quite so much and actually bothered to expand and engage in diplomacy.
The interesting thing about my second game is that the international situation was much more amiable than I'm used to in civilization games. There was only one major war, and five different civilizations were able to share a single continent without much territory changing hands. I probably would have gone to war with England if the game went on for much longer, despite their being a close ally, because they managed to build a high culture city near my border and it was playing havoc with my productive radius, but it was easier to just build the spaceship and win the game.
I think I may need to bump up the difficulty a little more. Usually the AI is a bit more proactive about ruining my day, and while I'm not particularly thrilled at the thought of my day being ruined, I've still got 16 hours to go, and it would probably be undignified to coast through it.