I myself played very little of HoI 3, biggest reason being that for me it was too messy and complicated looking. I think every grand strategy game is but HoI takes it to an extreme. However it was interesting to say the least, for me it was way too complicated and therefore I played it very little. The biggest difference to me and as far as we can see now is I think the aesthetics of the game, it looks inviting and fresh; just like EU4 did compared to EU4. For the rest I think Paradox went for the comfortable approach: they kept the good things: gameplay, interesting way of a world war developing and managing an army and they removed the "bad" things: complicated mess on your screen, uninviting looking. So yeah, hope that cleared some stuff up.