Welp, back to Civ4 BTS after a week of playing Civ V pretty hardcore.
Diplomacy is meaningless. The only things that matter for the player in terms of interacting with the computer player are the very simple AI behaviors that the computer follows. Those behaviors completely determine the nature of interactions between AI and player. The diplomacy screens is there only as a vestigial holdover from previous games, and basically serve no function. One huge side effect to the lack of diplo on this is that all the game's personalities feel the same when you are playing against them, which is a massive downgrade over Civ4 where each opponent had a real set of personalities and rules (bribe rules, unit probs, declare at pleased/friendly, etc.).
Seriously, every AI Civ plays the same way:
A) Spam cities.
B) Spam units.
C) If player is on same continent, ignore comparative military strength, wave death ray appendage and go "EXTERIMATE, EXTERMINATE".
D) If player is not on same continent, do nothing because oh god what am I doing here I am not good with navy.
Map scripts are busted, the game will gleefully bias you for certain starts depending on your nation (which is cool), but then not populate everyone's starts with some kind of military tech. No Iron OR horse? Yeah with the one-dimensional Dalek-inspired AI's, that means you won't be doing crap until rifles. Civ4 would 99% of the time ensure that nations with UUs requiring a resource have it close by. Here, definitely not the case.
Diplomacy victory is basically domination victory. Since Globalization comes so freakin' late in the game, by then the warmonger states have eaten up enough City-States to prevent ANYONE from winning via actual diplomacy. A diplomacy victory in Civ V is one where you beat the major powers in a late game war and liberate just enough city-states they puppted to get the win. Oh ,and have fun doing that through uncounterable nuclear weapons (SDI not in the game..were they out of their mind) that the AI has no apparent penalty for using but that you take monster diplomacy hits for until you are a world villain. Though not like diplomacy matters much there anyway. Also enjoy the tedious overseas micro that occurs since the game doesn't have any airlifting at all.
The game has some things that are clearly good steps forward for the base series-culture being tied into civics, the removal of tech trading (which was always a huge beaker multiplier in Civ4 for good players), balanced wonders that don't expire, slider removal, etc. The changes in the way that you can effectively iinteract with your opponents though just ruin it for me because they create a game that feels almost "on rails"-since the opposition in the game is so one-minded, and you have to interact with them on the higher levels (the game is actually much more fun on lower levels where the need to interact with your opponents is diminished, you basically just follow a set of hard rules adapted to whatever map start/nation you have. For me, that got stale after about a week.
Edit: Played a couple of more Immortal games today after reading some more threads on CFC, no matter what the following script plays out every time.
1) Usually go Tradition opening (it's just way better for the long game unless you are in a real cage match situation with 3+ AIs within like 20 tiles of you-this happened to me once), settle some cities, rarely in an AI's direct path.
2) Around 1000BC, one of the AIs will go into Dalek mode and instead of peacefully settling a spot near them-a far more productive option, given happiness issues, will declare on you from any standing (friendly, etc.), using the military units they get for free to try to zerg rush you with archers and warriors. They die terribly to an archer in a city+warrior defending. They will then get declared by another AI since they lost all their units, and that AI will usually start to run away with the game, so you basically have to try to get to the runaway AI before his city lead snowballs into something like 1350 AD riflemen (yes, saw that-and beat that-today).
There's an argument that the busted-to-all-hell Diplomacy model in the game reflects real players trying to win, but in reality as implemented here on high levels it's more about trying to ensure that the player loses.