So I will be doing a quick review of the game mostly in comparison to EU3.
The same (or with a few modifications), good or bad:
-you can still play all countries in the world at different periods in time and you still have different tech groups: Western, Eastern European, Ottoman, Muslim, etc
-Pretty much the same combat mechanics
-Pretty much the same map but with beautiful graphics (this is a warning too. If your PC can't handle CK2, it most likely won't be able to handle EU4).
-virtually infinite replay value and of course, easily modable.
What's been modified to a great degree? Good and bad:
-The trade system. Now you don't send nameless merchants to some trade node, now there are predefined trade areas and a trade node where you send your merchant to either collect or transfer money from that trade node. It adds strategy and tactics to the whole affair and makes merchant republics a lot of fun to play with.
-The technologically advancing system. It's now based on points. Instead of putting money into tech through a slider, you now get 3 kinds of power points: administrative, diplomatic and military. This is a huge system so if you want to know about it in depth, please go check videos that deal with this. It's a lot to it. I find the system to be good and bad. Monarchies get the ruff end of the stick when it comes to this system since monarchs rule till death and if you have a bad monarch, you're going to have a bad time. Republics, who have elections, generally get a chance to always have better rulers because if you end up having a bad one, you can vote him out the next election. But it's a good system. Really good. The flexibility and strategy it adds to the game is vast.
-The interface has been polished up and new features like core creation and culture swapping has been implemented to allow the player more input in the governing of his country.
-National ideas. Now here's a bit of a controversy. Many nations get personalized national ideas. It does add variety to the game but I don't know. Some national ideas just seem better than others'. For instance, Byzantium national idea really focus on mercenaries a lot, and if you don't like playing with mercs, you won't like playing Byzantium. But it's EU4. Not all nations start out equally and that's the joy of the game.
- There are general national ideas too, which you unlock at adminstrative tech level 4 7 10, etc. There are 6 administrative ideas, 6 diplomatic ideas and 6 military ideas each with several levels to it. So if you pick Innovative idea (which is an administrative idea) you will spend 400 administrative power points for each level of that idea that will grant you bonuses and when you complete the Innovative idea you get a complete bonus too.
I do find these ideas to be better than the way it was in EU3. Much better. But some ideas are flat out better than others I find. And some national decisions require you to adopt a certain idea and upgrade it to a certain level.
Anyway. This is all I care to say for now. the game is good. really good. It changes a lot from Eu3 but keeps a lot too. It's great. I recommend you buy it and you won't regret it. Also, Paradox is known for putting out several updates for each of their releases. I mean, Ck2 had about 5 expansions each adding a LOT to the game.