one unit per tile kinda scares me with regards to the combat system. It lends itself to very big bloodbaths where one unit take out another. The unit's counter retakes the square for the initial holder, the other unit's counter does the same for the attacker, and so on until the smaller army or the one not using combined arms well enough is expended.
If Firaxis wanted to make SoD's a thing of the past they should have referenced the forums here. There are any number of intelligent ideas into solving that issue from an ambush-fortify unit mode to a hard cap per square. But one unit per square with ranged archers over the top is not the solution. It just over emphasizes archers and takes away from realism in warfare up to the development of gunpowder, or more specifically, artillery.
But what's missing in these previews are what are the changes they're going to make to the economic model of the game. The role of religion? Diplomacy, etc.
The thing that is great about civ 4, is that there is great flexibility in the strategies you can use to win. I doubt the game designers had the notion of hammer based specialist based economies when they put out the game, but I think they should encourage that aspect of the game, lending to a deeper, richer strategic experience every time.
So far, all I've seen is some questionable choices in changing the combat system, with nothing new or exciting added [culture in civ 3, religion in civ 4]. Hopefully, previews in the coming months will serve to change my current opinion, but right now, this civ1-4 vet is only lukewarm to this new incarnation.