Victoria 2 is a fantastic game, and A House Divided really managed to improve the game.
However I didn't feel that Heart of Darkness was of much usefulness.
The Crisis system is a gambling feature. You have to say you're interested because you'll get a nasty prestige hit if you're not.
Then you see which side will give you the best odds of winning a war. For that you have to be one of the last to pick a side, and always after someone who's stronger or as strong as you. If you don't see a prey you can gang up on, you pass. If you still go for it, or worse, you are the first GP to side with a petition, you become the prey, and get devastated. How much sense does it make for you to have your country devastated because Albania wanted to become independent? None at all.
Multiplayer can exploit this even more.
So the Crysis system is a lot of fun if you're on the winning side, and it absolutely sucks if you're on the losing side. It sucks so bad that it's hard to find strength and continue to look after a devastated country. This is a game, not the "Weimar Republic simulator 2013"!
The improved naval system. I always run away from naval battles the best that I can. The AI is always faster than a human player and another human player doesn't have enough time to correctly counter a coming human naval fleet. In other words, the AI is always aware and the human players are never aware. I just want to deliver my troops to their destination and be done with it.
And one pet hate that still hasn't been fixed I don't know why.
When a country influences another and the pop-up shows up, you have the Go To button, which if you press it you have the map centering the screen on the GP which did the influencing. This is a completely useless feature. It should Go To the country that suffered the influence, so that you can counter that influence with some influence of your own. I have no interest in zooming into France after discrediting me in Egypt, I want to go straight to Egypt so that I can go to the Egyptian Diplomacy screen and counter-influence!
I also still struggle to understand the economy of the game. And there's still no connection between poverty and prestige. You can tax your people to death, they can have no money, no social services, and yet, somehow, you're more prestigious than other nations who provide better standards of living to their pops.
"Hey, they have secret ballots, free press and universal vote! Let's emigrate over there, vote conservative, and have none of our life needs met! And trade unions! It's the bee's knees!" (slight hint of sarcasm here)
And there's definitely a lot that can be learned from APD. They have a lot more decisions and event choices. It makes games much less repetitive and a lot more fun. Exploring new things is fun. Conquering the Xhosa as the UK by events is actually fun, because the imperialist in me wants to conquer them anyway, and it's only your pesky Infamy system that prevents me from carving the world my shade of pink! Mwahahacoffcoff.