Hello fellow forumites! You don't know me, but I do know you. Long-time lurker and Civilization player. This is my first post, so decided to introduce myself a bit.
On topic: many awesome things already were said, won't repeat any of these. Will add my 2 cents, though. Think I didn't saw them anywhere in this kind of posts (at least 3 big ones at the moment).
1: Let's imagine we are playing Earth map, for simplicity. I am playing... for example, Russians.
And WHY in about 2000 BC our Civilization, located in Eastern Europe / Western Asia (I tend to expand to Europe first, no matter), so anyway WHY do we already pretty familiar with Zulu, Spanish and Chinese people?
My point: exploration should be changed. Fog-busting lone warriors, traveling for centuries and millenias are silly. It should be HARD to travel through all Eurasia or comparable landmass.
2: Again, Earth map. I am... for example, Egyptians. It's about 1200 AD, and things are going smooth. Suddenly, my friends Chinese (well, okay) are calling me to declare WAR to their most hated adversaries, the Vikings. I mean, huh?
Dear Qin, you are in China. And Mr. Lodbrok is in Norway. And it's middle ages, we don't have motorized transportation, railroad or airplanes to get there! And by the way, WHY you don't like Ragnar? His beard is better than yours? You are both Buddhists, so NO, this is not crusade.
My point: AI should know where he is and where are his neighbors. It is pointless to declare war to Scandinavia, when you are living in China.
3: Earth map for the win. Persians this time. Our neighbors, Babylonians, are our good friends. We share Hinduism, some of other folks nearby are polytheistic too. All is well. I perfectly understand why Babylonians declared war on those heretics, the Egyptians - they are Buddhists, for gods' sake! Holy war, purging of infidels and all that stuff.
But why we care about Aztecs and their American vassals? So what if they are Jewish? They are on the another side of the planet! I don't care about their religion, and my dog neither! Crusades were fine (well, in game terms) - they tried to recapture their Holy City, big profits! But to block trade with Aztecs just because they don't believe in our 31415 gods? Who cares, give me my Corn and Gold!
My point: Religion issues are nice, but see #2. They aren't THAT bridge-burning, and nobody cares about religion differences with people from the other side of the globe. Neighbors - sure, these bastards should know better.
Edit: and something else, not unheard before, probably.
4: Some mods for Civilization IV are amazing. For example, Revolution Mod (and FfH, which I am hooked on badly, but that is another case). Revolution Mod is awesome. Something like this should be implemented in Civ V by defauld, and I think there are a lot of other mods which concepts should become a new default rules.
I believe that Firaxians are far from being arrogant snobs, ignoring the community and its' best ideas. I also firmly believe that any Civ IV modder, whose work will serve as a basis for some Civ V feature, will be only proud and happy.
5: More about mods. The one thing which is troublesome now is that only ONE mod can be used at the same time. There are Mod Mods, and sometimes people do merge mods, like in Rise of Mankind + Revolution + Dave's Combat Mod case, but it is not really handy.
Check out Morrowind / Oblivion, where you can install loads of various mods at the same time, improving your gameplay in different areas. Much better core design principles. Civilization V should use similar model, because I want to play FfH + Revolutions (just for example, actually I don't want to, because I am playing on huge map with all civilizations at the same time).