The map, the portraits, the artwork... style- and gameplaywise Hearts of Iron 4 looks like the exact opposite of HOI3. And that just doesn't work for me (and I'm sure for others neither).
It's an immersion breaker to see your giant toy-soldier-troops doing push-ups while reading an event about the spanish civil war, or the battle of Leningrad. And immersion was a huge factor that made me play Hearts of Iron 3 in the first place. But from everything I've seen so far this game is a lot less realistic ("realism isn't a meaningful argument blah", I know).
Now we have political power, Hearts of Iron 4's mana points that you can spend either on promoting officers or puppeting greece (realistic, right?). And we got tanks and fleets that run without oil. Than there is research, world tension, ministers... all things that looked cool for me in the dev diary first, but when I thought about it again I noticed they were more or less better/more complex done in HOI3.
Another thing that is just off, is the map. it looks like an EU4 map mode with completely overblown effects. Why day-night-cycle? Can't remember anyone asking for it. Why creating all those unnecessary animations and force people to use them, who simply want counters? Why bright neon colours and huge letters with white outlines instead of simple names (the Soviet Unions "S" doesn't need to be larger than Spain...). Just to remind you, historical maps looked more like this (1944):
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So far the only new things that I really like are equipment and the more realistic manpower pool. The rest is different, but not really that much better for me.