Glad you guys like it! As for Civ5, we'll see ... it sure will be interesting to see what's different. The conventional wisdom for sequels is 1/3 new, 1/3 updated, 1/3 the same, and with the change to hexes and the strong hints about combat changes in the press release, I'm not sure how much space there is left for major new gameplay systems.
Of the two, I think it's possible Firaxis will have something in the vein of BarbarianCiv in 5 ... I don't think they're into new civs appearing mid-game, but there was a mechanism in the code for 4 to allow "minor civs." So, I have expect some form of minor powers that fill in space between major powers and create more interesting diplomacy/war options early in the game before the major civs come into conflict. I also think the barbarians will get updated, some way of taking the barb uprising events from BTS and making them more organic/less random.
The odds of some form of civil war/revolution mechanics in vanilla Civ 5 are basically 0 I think ... they might go that way in an expansion, but even there I haven't seen any indication it's on their list. For a developer it probably doesn't seem like a good investment of resources, as they would need to make the system easy and gentle enough not to discourage first time players. Given that most players will only want to deal with revolts occasionally, it probably doesn't seem like the best investment of time for Firaxis. That's my sense.