Ok, first impressions.
Most importantly, I want to emphasize that this game has zero NEGATIVE new features(ie features that would "ruin" the game for the Civ veteran). There weren't any drastic changes to freak out over. All you'll find is the same old Civilization fun in a much newer, more refined, prettier package. Great stuff!
On the surface(granted I haven't gone past a couple of hours of playtime - I haven't fought a battle against another Civ as of yet), the change that I found most notable is the depth added to square improvements. The days of running around planting roads and irrigating your land are over. The "irrigation" option has been completely removed and replaced with a myriad of new options including Pastures, Camps, Mines, Net(fishing), Plantations, Windmills, Lumbermills, Forts, Workshops, Winery's etc. etc... In order to harvest squares with horses, cows and/or elephants, you need to build a pasture. In order to harvest grapes, you need to build a winery. Very cool and warmly welcomed on my behalf.

Sometimes, say for instance you build a cottage in a forest, it doesn't immediately destroy the entire forest.
Fun new things include:
Great people - I've had 2 great engineers and one great artists. They can be used in several different ways including building techs, speeding up production and in the case of the artist, I was able to have him produce a "Great Work of Art."
Killing Aminals

- Lions, Bears, Tigers etc play a role similar to what the barbarians in CivIII played in that they are really just good for familiarizing yourself with the combat system earlier in the game. They do roam freely and may have the ability to attack units(although I didn't experience this).
Off of the top of my head, that's really the only noticeable features I found. The animations and the added depth are all there but not worth mentioning until I've explored them fully.
My Setup:
ATHLON 64 4000+
40 GB Raptor
GeForce 6800 GT
SB Audigy 2
Windows XP Pro
No crashes in around 1.5 hours of playtime.