This seems to have some interesting iterations on the FOG:Empires design. The resource & trading system is more detailed as resources are produced in quantities of 1-3 instead of being infinite. A single Farm can no longer provide Grain for the whole province. Trading also is more affected by Acumen, which allows further away regions be prioritized when determining where a resource ends up as a trade good. This leads to an interesting implementation of medieval Merchant Republics that really punch above their weight commercially with their sky-high Trade Acumen. Generally, fiddling with the buildings and resource chains is as satisfying as ever but the overall commercial environment is more dynamic.
Combat seems to be similar as the last time. I think the unit supply, ranged support, general bonus/malus etc. systems are similar as in Empires. Raising an army has a bit more depth with different Standing Army / Levy / Mercenary categories, with their corresponding advantages/drawbacks. There also seems to be more variety to the Regional Decisions, which are a nice, chunky way to influence the realm.
The overall theme of consolidating the realm against internal division and external peers works well. You have to build tall to expand wide with the Influence and population mechanics working as a handbrake on overexpansion. Overall, Kingdoms is a small but consistent improvement over Empires.
There are downsides, however. The Grand Campaign has an obscene amount of minor nations on the map and it is quite slow to plod through turn-by-turn. The UI I think is actually worse than in Empires, with an odd gray-on-gray text scheme, almost unreadable realm names, and indistinguishable heraldic icons here and there on the map. Thankfully, the last two points can be reconfigured in the settings. The diplomacy UI is an atrocious clickfeast, and considering how important the trade good chaining part of economy management is, the tools to dig into which resources are getting shipped where for what reasons are painfully lacking.
The character system is also underwhelming. They have variable skills, sure, but the dynastic line doesn't seem to lead to any court intrigue or drama. Low loyalty characters are total liabilities, but I don't think there's any downside just keeping them away from positions of influence even if one of them is your firstborn son. There's just so little to DO with the characters, and very limited ways to interact with them.
The patch release rate for Kingdoms is still quite rapid, and the main dev is obviously a scholar, gentleman and a committed maniac who'll be grinding out balance tweaks and performance improvements as long as possible so in all likelihood the game will continue to get more refined over the near future.