Summery: Choices is what makes a game fun. EU4 doesn't offer any choices to the player whatsoever. Therefor EU4 = not fun. Gather some friends and play Risk instead.
In my experience EU4 doesn't present the player with anything meaningful whatsoever. During the entire run of the game, I do not feel like you have to make even one meaningful choice. All that matters, and all you can do, is conquer. Aside from this, there is nothing. Sure, local autonomy, vassals and marches do help you organise your conquest, but don't take away from the fact that all you can do is conquer, conquer and conquer some more. Now this doesn't have to be bad, there are plenty of games which revolve around this after all (HoI being one of them). These games just offer you a choice as to how to go about conquering. In EU4 there is no tech tree, there is no (meaningfull) difference between units and there is an optimal compositions to win with. There is no choice, no variety, and no reacting to changing circumstance. They say war never changes, and in EU4 this is certainly true. Throughout the 400(!!!) years of this game, you will *always* be doing the same damn thing. Look at StarCraft 2: Every faction requires a different play style, every faction has multiple equally valid play styles, and every opponent requires a different play style to defeat. Not to mention that the game requires significant skill to be good at. Do we see any of this in EU4? No, it is all the same for every damn single country, and there is no skill to it. There are optimal builds, and there is no skill to building them. Look at HoI. Often I find myself having to make a tough choice; do I build tanks or battleships? Not because the game limits me to one or the other, but because my opponents require different approaches to defeat them. In Vicky I often find myself having to accept a slight at my authority not because I couldn't win the war, but because winning the war would be devastating to my country. Have you ever found yourself facing any such (meaningfull) choice in EU4? I haven't. Conquer, conquer and conquer some more. That is all to it.
The tech 'tree' in EU4 is perhaps most symptomatic off it all. There is absolutely no choice. You simply push the button and you are better. In Vicky you have to choose. Will I focus on my army, my navy, my people or my economy? And each choice is equally valid. It just depends on what your circumstance and what you want to achieve. In HoI it goes even deeper than that, with you being forced to choose between different aspects of each branch. In short; you are always trying to do the least you can while still being competative.
Have you ever felt like you had to make a choice in EU4? I certainly haven't.
EU4 also feels extremely empty to me. Vicky and HoI offer such a good feel of the period they represent. EU4 could be set during the iron age, and I wouldn'thave noticed the difference. It could be set 500 years in the future and I wouldn't have notice the difference. Because the game rejects any form of railroading, the events of the game feel meaningless. Aside from a few events, you could replace the complete text of the event and it wouldn't matter. It would still be the same event, and still have the same (meaningless) effect on your country. Sure the HoI might be highly railroaded (why play a WW2 game if you don't want to play WW2?), but at least they capture the feel of the period fairly well. Vicky 2 leaves a lot to be improved upon, but the events capture the period so well. The rise of liberalism, socialism, communism and facism, it all makes sense in the game. In EU4 I don't get any feel for my nation or the period whatseover.
Lastly, and I don't know why, but pretty much everything the entire look of the game feels wrong to me. Starting with the interface. The graphics style is offputting to me. Blue and gold is just... no. I know it are the Swedish colours, but... no. The map too just feels wrong. 'Play doh' has often been used to describe it here on the forums, and I agree. The terrain map mode is okay, but when I turned to any other map mode, I really just want to quit the game.
I had one really fun game in EU4: playing as Austria as the HRE. Trying to keeping the princes in line, the electors happy and the French and Ottomans off my back. Now that was fun. I had to choose: Who do I fight, what for and when? A lot of challenges to my authority had to be let go simply because I couldn't afford it. I made one wrong decisions, and paid dearly for it. That was fun! But once I revoked the privilegia, the game got boring really quickly. I beat France, the Ottomans and Russia at the same time without moving a single army. So much fun...
And yet, even this HRE game relied on only one thing: squashing my enemies with armies. There was little more to it than that.
I did decided to give EU4 one more go tonight. Started a game as England, think it starts out at war, lots of unique events, and plenty different paths to follow. I quit about one year in. I quickly realised how boring wars are in EU4. Either you are vastly superiour, or you draw the ai into a mountain province. Once I realised this is all I'd be doing for 400 years, I gave up on the game.
The only thing I consider fun in this game is doing incredibly silly things. Like trying to turn the pope protestant (don't know if you can, cause I gave up on the game before I got around to doing it). But those things are only fun once. If the only things which are fun in your game are; a) doing silly things, or b) breaking the game, I don't think it is really a good game.
Sorry for sounding so sour, but this is how I feel about the game.