I would suggest that the problem is less that the AI always attacks you- anyone who played Civ2 had more than their fill of it- but rather that there's not enough balance between the AIs to keep the warmongers from eventually rolling over their neighbors. Almost every single one of my games (on Pangaea, or other land heavy maps) has followed the same pattern, whether I REX early or not: everyone plays fairly nice with each other up until the late Medieval era, then one of the militaristic AI (often Bismarck) slowly starts grinding up neighbors and city-states until they control half or more of the world. Even the other military AI have trouble slowing them down, as I've seen Bismarck just crush Caeser and Monty in different games, in situations where the smaller civ still had a fair amount of resources.
So the issue is that war isn't just the only option for the human player, but that it's the only successful strategy for the AI as well. When every end game- or at least a lot of them- are going to involve a late game human vs AI showdown, there's just not a lot of reason to avoid getting an early leg up on the eventual competition.
I'm not sure what a fix would look like. Different AI to AI diplomacy? Different mechanics for CS getting involved in AI vs AI wars? Allowing CS to expand to two cities each, so that all that empty space on the map doesn't go to aggressive AIs? Just don't know.
I completely agree with this, especially the "not enough balance to keep warmongers from defeating their neighbours" part. I've played a few games on Huge maps (my favourite size since Civ 3, at least) and one or two civilizations just keep grabbing more and more and more cities. Who cares if they have incredible amounts of unhappiness? My small, cultural (and often very scientific, too) civilizations stand no chance when they are attacked by a huge empire that usually has units one tier more powerful than mine.
I think introducing a few or all of these changes would be more than enough to make non-military victories possible:
1) Make AIs better defenders. On a map with numerous civs that can defend their borders no one will eat up all the cities and send 200 tanks to my borders.
2) Civilizations should be reasonable in their alliances and animosities. A civilization that eats their neighbours one by one would be stopped if other civs realised the danger and teamed up against them to take them down a notch. It worked in Civ 4, it COULD be implemented again.
3) Unhappiness penalties should go beyond just a hefty penalty to production and military. It's simply not enough. Past a certain point, dunno, additional cities should stop working altogether? As it is now, the "can't build Settlers anymore" just ENCOURAGES civs to attack more.
4) Small civilizations could use a way to make their cities much stronger. I think bringing back cultural defense would work like a charm. Small civilizations tend to have incredible culture by the 19th century. If it added them 30+ points of strength, it'd be possible to defend them with a relatively weak garrison.
Really, I love the changes introduced into Civ 5. It's a great game... for multiplayer. I don't share many of the common complaints about the game - overpowered and money-based city-states, cities too weak, 1upt... it all works very well... but it is easily broken by the fact that one AI always dominates the map.
I'm hoping this will get improved eventually, but frankly - I'm counting on mods more. But so far, I don't like that most mods change things a little too much for my tastes. I'd prefer the same game with a tweak here and there... and modders try to rebuild game mechanics in ways that don't convince me. Am I alone in that?