Hi all, long time lurker first time poster. Like many here I've been losing a lot of sleep with G&K over the past few days, so I'll share a couple thoughts:
Playing Celts, Continents, Prince, Epic.
Just got to spies, and while I agree they don't have as much variety as we might have hoped, I'm really enjoying the way they work. The Civs on my continent are spaced out, so everyone is friendly due to no border conflicts. Iroquois want my land, and Polynesia isn't happy about me building so many cities, but no other negative modifiers for me.
Then we hit the Renaissance and Spain comes running: Hiawatha is planning to attack. I'm not surprised, I do have a nice city in the center of the continent parked next to the Grand Mesa that holds all of the banana along with some horse and iron, plus the bulk of my army is overseas intimidating and attacking some city states. But things get interesting on the next turn. My spy reports Kamehameha is mobilizing to attack me! Sure enough, I check the diplo screen and both nations are now neutral, want my land, and think I'm a menace. There is no doubt they have teamed up against me.
Now in a regular game, I would have been caught with my pants down. With the heads up from espionage, I quickly make some defensive pacts, buy some more units and call my army back. After this is done, a few turns later, Poly has swung back to friendly. Later still, the same happens to Iroquois. I think me prepping for an attack made the AI reconsider and then cancel it altogether. The spies didn't just help me ready for war, they helped prevent it entirely. Pretty cool stuff.
A few other observations:
-- If a city-state is angry with you, it seems they can't be bullied. I had warred with one city state and then made peace, but even with my entire army parked in their territory the game said they weren't afraid and would reject demands. Then I randomly fulfilled one of their quests, improving their modified from -50 to -20. Their screen then listed them as "Afraid" and I was able to bully them.
-- Civs finally offering to continue trades has me over the moon. I absolutely hated this about vanilla. It was infuriating to lose open borders and be kicked out of an ally's land in the middle of a war where I'm defending them. A major flaw from vanilla that I'm glad to see fixed.
-- Like others have mentioned, if you're going to get deep into the religion game, be sure to pick at least one modifier that lets you buy things with faith. After the spreading phase is over, you'll start raking in faith points and will need stuff to spend it on. I've got Pagodas in every city and have built my entire army out of Holy Warriors.