If you never played with some of the AI mods you should look into them as they make the game much more challenging at lower levels without cheating.
That said here's an interesting post, I don't know how legit it is:
http://apolyton.net/forums/showpost.php?p=5753192&postcount=23
Reading the Civ5-thread in Paradox Interactive's OT, a Danish guy has written down some info given in a Danish magazine:
Quote:
- Switch from squares to hexagons changing the way the game plays. More room for maneuvers and more tactical options.
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Changes to combat. More depth in combat, no more stacking of units. This will lead to bigger focus on terrain.
- Inspired by Panzer General.
- Reintroduction of Bombardment, now archers and siege equipment can shoot over melee units.
- Better diplomatic AI.
- More diplomatic options between players.
- Less "cheating" AI.
- Religion is not a factor anymore.
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Ressources are not infinite. For example one source of horse only supplies enough horses for 1 unit, but when that horseman dies the horses will respawn as a unit. (this confused me alittle, i guess we will have to watch it in action)
- City States as a sort of small countries that never develop beyond their single city. They can provide bonusses if you befriend them, or you can take over their land.
- Civics are out, now there is something called "Social Policies".
- About the same amount of wonders, the tech tree will feel familiar. Great People still in.
- Some victory conditions changed. For example in Conquest you only have to capture all the other capitals. Eliminates boring mop up phase.
- Unique Civ leader bonusses, no more standard "Spiritual" or "Financial".
- DirectX 11 support.
- Built in webbrowser. Sid Meier is also working on a facebook application of Civilization.
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"- Less "cheating" AI."
It looks like religion has been taken out
City states sound pretty interesting as does ranged bombardment and archers.