Below is the answer Jon Schafer gave when asked about the biggest differences between Civ4 and Civ5:
There's a few things. One of the big ones is that, instead of square tiles, we're going to have hex tiles. There's a couple things with hexes that give the advantage over squares. First off, graphically they look better, because no tiles share corners. You can have more of an organic look. You don't have to have 90 degree angles everywhere, you can have it more free flowing. It also helps out with gameplay. Everyone who plays Civ knows that when you move your units, or whatever, visibility range, you generally want to go along diagonals, because you cover more distance that way than if you go a straight line. With hexes that's no longer a factor, so there's benefit in gameplay and in the visuals.
Couple other things. We’re really excited about the changes we’re making to the combat system. Anyone who’s familiar with the old SSI [strategic Simulations, Inc] games like Panzer General will recognise the change that we’ve made. That’s the inspiration behind the system. We really liked the mechanics in that game, so we took the basics from it and iterated on it.
Some of the details on that is that instead of having big stacks of units like players are used to, you’re only going to be able to have one military unit on each tile. So you’ll see fronts form on the battlefield more, instead of these big masses. We’re also going to be introducing ranged units to the game, like archers and catapults. They’ll be able to fire over multiple tiles and hit enemies away from them. We think those things will make the combat really exciting.
Origineel Artikel: http://forums.civfanatics.com/showthread.php?t=356818&goto=newpost