Quick input from first games with 1.21:
1) Courthouses work great!
2) even without courthouses corruption is still bad, but waste is somehow smaller than before
3) AI-AI trade to 100% on Regent works just fine
4) made ancient techs more expensive, also reset map size factor for tech development - works fine since you don`t get the point where you develop Gunpowder but because of landgrabbing noone has even built a Temple yet

BUG here in the AI, needs to be addressed!!!!
5) Combat a lot better with 1.21: lots of units everywhere means lots of fights. Finally the sample is so big that freaky occurences don`t matter too much. They happen, but often enough, and to both sides, that they even out!
6) Fun-factor is a lot better since now different playing styles are possible. You can flood the world with modern units when the enemy is still in Middle Ages - but then you`ll have weaknesses somewhere els. or you can flood it with lots of ancient units - and enounter modern Armour! Or you can out-culture the enemy. Or outexpand. Or whatever you like!
7) Culture flips work better now. maybe that came from 1.17, I never tried that, but now I can plan on a flip to happen and it usually will in an acceptable timeframe.....
all in all:
1.21 needs a few little fixes, but they can be done in the editor. Otherwise:
