I guess to put it bluntly, the civ designers have gone too far with culture. In my opnion they probably made borders depend solely on culture because they didn't want people to ignore this feature of the game. (That might be a similar reason for why they've programmed rival civs to constantly bombard you with spies - to make sure you're not ignoring the espionage game.) When you think about it, if borders didn't depend on culture by itself and there was no culture victory then culture would have had no value. From a design perspective I'd argue it's undesirable to hinder other aspects of gameplay to introduce a new feature.
Military being the most important property of your empire has been there since Civ1 and I guess IMO it's a shame that's being dwindled away slowly. Don't get me wrong - I don't want the game to be all military or even anything remotely like Civ1 - but I think they've tried to borrow too much from the SimCity Societies type of game rather than the Simcity2k gameplay. In other words, trying to make the game more aesthetically appealing doesn't necessarily do much, if anything, for gameplay.
The depth of gameplay and the interesting choices don't just depend on the number of different ways you can spend your resources. It's more important probably to have a variety of successful playstyles with fewer features. It's a bit hard to describe what I mean so I'll leave it there before I bore everyone to tears...
You aren't boring me at all. I agree. The idea that there should be borders of some kind is right on. That the "culture" border does what it does is more of a problem than a solution--I agree. In fact, I am always disapppointed to see that the %'s of a given tile are culture based and not race based. Wouldn't it be more interesting to have the two borders you mentioned--political and cultural, and to have the interplay work or realistically? In other words, while an English tile is overawed by the neighboring French culture, this does not make them French. What it does do is what Religion does in the game(something I really like) and that is, create a stronger bond between two civilizations.
Having the distinct borders would mean that war between the two could potentially erode that cultural influence without neccesarilly affecting the political border.
It might also introduce a kind of "fifth column" aspect of relations, such as when, during the French Revolutionary and Napoleonic Wars there were distinctly Franco-philic Englishman as well as Franco-phobic.
I agree, however, that putting too much influence on any one aspect of the game can throw it off.
On that note, I am not entirely happy with how trade and commerce operate in Civ4. The trade links are completely random for a given city and do not seem to be based on much of anything. It would be far better if they took the resource supply/demand aspect of Civ2 and used it here, only automating it the way they have done(though poorly) in Civ4. I say this because economics has historically been a far stronger force in any civilization than culture has, though not always as strong as the military. A civilization game that correctly balanced economics, politics, and war would be best, obviously, but doing so would entail some serious structural changes to the game.
Here's one I have been mulling over:
How necessary is a specific "leader" over each civilization? Is there a way to not only show a varied progression of leadership, such as when there is a revolution, but as for your civilization to splinter or simply fall apart? If those three aspects of the game played out better, you could be fending off other civilizations and new ones who have left your original. Imagine a break-up where you have to choose which of the three or four pieces of your former empire you would then control?
For this to happen, culture would have to be factor--that is, a city whose culture far exceeds that of any other of your civilizations cities would actively begin to tear away.
Maybe I'm getting too deep on this, though. I read a lot of world history and I see moving and shaking that is simply not represented by the game in a realistic way.