One of the most noticable differences in Hearts of Iron III has to be the game's new design focus. In previous Hearts of Iron installations, in-game provinces could be hundreds of kilometers in width, and whole corps would be moved around with a click of a button. Hearts of Iron III's new design focus has been focused downwards -- for example, the amount of provinces that you'll be fighting over has increased by what seems to be a factor of four or five and frequently, in even the most fought over fronts, provinces will be held by a mere two or three brigades of troops. Fairly streamlined affairs such as diplomacy and tech research have also been bolstered in complexity. In some ways this is a positive boon to the player -- diplomacy now gives players more options, allowing for countries to purchase licenses to build units they might not otherwise be able to produce or giving Axis members the ability to declare limited wars. The amount of things that need doing in Hearts of Iron III can be a little daunting, especially at first, and have also included the ability to allow the computer to take over just about anything to allow you to focus on specific issues.
Not everything benefits from this increase in complexity though. Indeed, in many ways parts of Hearts of Iron III have arguably become worse than in previous installments. Take research, for instance: in the second game, nations had one to five research teams with which they could research things such as new battleship or tank designs. It was simple, yet still allowed for both major and minor powers to research effectively. This system has been done away with for a more complex system in Hearts of Iron III, where players now research component parts to units such as tanks. Where in the previous system you'd research the tank itself, you now research its engine, gun, armor, and reliability. Moreover, each nation can now research only so much as its leadership points allow -- an all encompassing resource that needs to be divided between not only research, but also spy production, diplomatic action points, and officer training. This is all well and good for larger nations, but smaller nations are now generally penalized to a greater degree than was ever seen previously and tend to lag far behind their historical counterparts.