Not the best in the Civilization Series
I really, really want to love this game however honestly Civilization 4 is a bit of a disappointment for me. Although I prefer it to Civilization 3 it certainly was not able to regain the magic that was Civilization 2. Why? It's very hard to pin point exactly why but please let me try to explain.
Honestly it's hard to compare - Civilization 4 definitely gives a better graphics (3D models), fully more dynamic diplomacy system, extra features such as religion etc but the basic premise stays the same as the older versions of this game.
However I honestly think they have overcomplicated the game to the extent that it's just not as much fun as Civ2. Culture while good (was in Civ3 also) doesn't Allow cities to be built inside enemy zones of control without causing war - while this is probably more realistic it lessens the gameplay or at least makes it more difficult to expand. This is a more difficult AI alright than Civ2 but not necessarily a more fun one. Needing Resources to build certain units is a nice addition but frustrating at times as it depends on luck if a resource is near you or not at times.
I think it would be a good idea to allow pre-game options to change these rules to old style if required / culture/no culture option, special resource/no special resource requirements etc.
System requirements and patching - make sure you get the latest patch for the game otherwise it will be prone to crashes and freezes and all other annoying buggy things. It used to take forever to load before a few patches came out at first. Also you need quite a powerful graphics card (at least relatively speaking 3 years ago when this was a new game). Why do developers always try push the graphics to the lime - that's just annoying that you need to update hardware continuously to satisfy developers whims). This is a turn based game - it's generally like because of gameplay not graphics, honestly 2D sprite would have done fine.
Modding - now this is a big issue for me - I'm not a 3D modeller. They say that Civ4 is meant to be one of the most moddable games ever. However sadly in practice this is not the case. Let me explain: Back in the days of Civ 2 using non-animated 2D sprites the technology was relatively simplistic to mod a game with user defined graphics - the simple Paint program supplied with windows was enough to create your own units. Name descriptions and attributes for units could be updated easily in text files. The consequence was that very creative people with minimal tekkie knowledge were able to create their own mods and scenarios for the game. The result was that there was an abundance of user made mods and scenarios available on the web. Some of these admittedly were quite bad but there was a lot of gems there too that were better than any of the commercial scenarios bought in expansion packs for the game. Sadly all that has disappeared with 3D models - you need skill at 3D modelling and animation now and vaster programming skills to make any worthwhile mod at all for the new game. Now there are a lot of skilled modellers out there but this doesn't always go with creativeness. I'm not saying that there aren't good mods out there - I'm just saying the consequence of the 3D model graphics is that it narrows the band of people who can contribute mods and scenarios that might have good ideas.
My complaints are a matter of personal taste, some people absolutely love this game - I enjoy it from time to time but never got hooked like I did with Civ 2. The magic is lost I'm afraid.