This post will probably only interest two people (Jess and my bro), but I'm gonna talk about it anyway.
I rarely buy computer games, the last game I bought that wasn't a Civilization game was Black and White. Needless to say, I don't play a lot of high graphic PC games. So when I bought CivIV, I learned that my computer, after 5 years, was out to date. That's a whole other story, but needless to say after some video card and rdram installing, the game is working great.
I've played a few games so far, and heres what I think:
Leonard Nimoy is sweet. Spock himself narates the game, and there are nice little quotes from throughout history for every tech you learn.
Religion is weird. This is the biggets change in CivIV and I'm still getting used to it. There are 7 religions, and they are all the same, its a matter of when you learn them. The goal is for you to try to get other Civs to adopt your relgion. The more you spread your religion, the more people like you. I haven't gotten to where I really know what I'm doing yet. I don't find myself trying to make missionaries to spread my religion (so far I try to get one early, to get it out of the way). If another civ adopts your religion, you can see into their cities, which is kinda cool. But I'm not very aggressive, I don't really start a lot of wars, so I don't particularly care what people are defending their cities with.
Bring back the Great Library! They switched up some of the Wonder attributes and I don't like it. The Great Library now gives you 2 Scientist specialists in the city you built it. The old attribute (gain any tech that two or more known civs have) is now with The Internet, which is practically useless (although, I do like that its icon is Al Gore). By the time I have The Internet I already have almost all the techs or I'm at least the most advanced.
Other Wonder stuff:
The Pyramids don't give every city on the same continent a granary any more. It now enables you to access any of the government civic functions. Honestly, either one doesn't really make sense.
There is no Darwin's Voyage Wonder, which sucks. That was a great Wodner to grab some free technology. As one poster said "They stuff the game full of religion and take out Darwin? Where was this game made, Kansas?"
Wonder Videos. Still not at cool as the CivII videos. They just aren't. Speaking of CivII stuff, still no talking advisors. Those guys where sweet. I almost never used my advisors in CivIV (or at least my advior screens).
Still no Hitler and where's Abe?. There are a ton of leaders in CivIV, the most I think to ever start a game (At the end of CivIII, after all the expansion packs, there were more total Civs and leaders). Some Civs have two leaders, which is kind of cool. Cause a French team led by Napolean has a little bit more aggression than one led by Louis XIV. The American are one of the civs with two leaders, FDR and George Washington. This is the first time the Americans have been Lincoln free, and I kind of miss the lanky bastard. The Germans also have two leaders, but neither are Hitler. I know, it probably wouldn't be the most PC thing to do to put Hitler in the game. But they have Napolean, Genghis Khan and Mao Zedong. These aren't exactly nice guys either. Meh. Also, would it kill them to have Churchill for England?
One final leader note, their clothes don't change. I know its a small beef, but since the origional Civilization, the more advanced you became, the nicer your leader dressed. When you start the game, everyone is in rags and by the time you ended you were wearing a suit. Now you start a game and Bismark is in full uniform with a metal helmet. Bah.
Workers really work. There's a ton of stuff for the little buggers to do, you can buld farms, pastures, quarries, mines, towns and a million other things. I'm not smart enough to have them do it all myself, so I just put them on automate and let them go. They don't seem to like to chop down forests as much as in III, but its still a problem.
Resources matter. There's tons of more food stuff you can trade. And you can mine marble and stone. If you get these, you're set to build some Wonders. Certain Wonders cut their production time in half if you have marble or stone. So it's good to explore early and stake your claim to the valuable land.
Settlers are different. First of all, they can move 2 spaces a turn, which is cool. But the bad thing is, there are now wild animals that attack your units, so its imperative that you send the settler out with a defense, or theres a good chance they will get eaten. I find myself having to actively push myself to make more settlers. I seem to lose track of them in my race for religion and wonders, to the point where I only build 4-5 cities. Which is bad, cause some wonders require 6 buildings to construct (6 of you cities need a University to be able to construct Oxford University).
Map Size/Game Speed. I find the default map a little too constricting. I don't mind competing with Civs for land, but I don't like dealing with 4 on a rather small continent. I like larger maps, more space, more resources, more room to let me just horde Wonders.
My cheap attack mode doesn't work any more. My CivIII tactic was to get an Right of Passage aggreement with a civ and then load my military onto their land. Once I have all my guys situated, I attack in one giant turn. I tired that in CivIV, and once you declare war, all your units move off their land.
Speaking of war, I had a tank get killed by a defending Longbowman! What the fuck. Can't we fix this shit. The longbowman is the best defensive unit in the game by far. They remain relevent for a long time (they can even stop tanks here and there).
UN Victory? Does it work? I build the UN Wonder, we have vote to see who should be UN Chairman. I have enough votes to get it, but nothing happens. I dunno, maybe I'm seeing somethin wrong (not everyone voted for me, but I'd get 500+ and I'd need 423 out of a possible 600+... I dunno)
Inside jokes/cool stuff. As mentioned with the Al Gore/Internet Wonder joke above, there are some fun Easter Egg type things. There are a ton of Great People (prophets, artists, scientists, merchants etc) and they each get unique names from throughout history. These are just cool to see, Shaskpere, Plato, etc. Fun stuff. The quote for the Rock and Roll tech (and the song for the Rock and Roll Wonder) is by the Velvet Underground. Cool stuff.