Some of the biggest changes have been wrought in diplomacy and economics. The new list of resources is steel, energy, oil, rare minerals, supplies and money. The old list didn't have money, and had coal in place of energy and rubber where rare minerals are now. The new design gives more emphasis to resource areas other than the East Indies, where rubber was formerly most common. Rare minerals now envelops all the specific resources that it's difficult to engage in modern industry without, such as tungsten, graphite, and bauxite/aluminum. Many countries are more playable thanks to these changes, since few had access to rubber or oil in the original game.
Trade is streamlined as well. There's no longer a guaranteed-loss world market where at best you could hope to exchange 2 of your item X for one item Y, meaning that even that with a near-monopoly on oil you were guaranteed to lose out in a deal for steel. Now, countries trade directly with each other. This makes alliances tighter, and gives a fair value for your products. Energy isn't worth much, but rare minerals and cash are. The game includes a handy list in the diplomacy menu that shows who has surpluses of what good. If you appear in that list with everything and your industries are going full steam, you're gravy.
There are far fewer divisions - especially air divisions - and you're not as reliant on special hardware like mountaineers or paratroopers as in the first game. Armor is more limited too, because of fuel considerations. Motorized infantry scarcely seem worth the fuel, unless you're the United States. In a nice touch, the player adds brigades manually to his divisions, since they're built separately. You can queue up any number of brigades, divisions, air squadrons or naval squadrons, and the interface will let you know if your current production levels can support that. There's a bewildering variety of brigades, like armoured cars, heavy tanks, artillery, anti-tank guns and anti-aircraft guns.
Resupplying and reinforcements are handled automatically as well, through the economy sliders. Be careful though, there seems to be a (bug?) where, in winter weather, your economy sliders will suggest the need for massive reinforcements when in fact your troops are taking little in the way of attrition. This can and will quickly deplete your pools of manpower. Manpower, more than oil, rare minerals and supplies, limits your nation's ability to fight in the long run. It replenishes very slowly - 1.35 manpower per day was the best I got from my German campaign. Considering that two dozen infantry divisions, and a dozen armored divisions, plus brigades, will spend roughly a year's worth of manpower, it's vital to be careful - and makes the loss of entire armies due to encirclements (like what happened to Germany's 6th Army in Stalingrad) absolutely devastating.
Remarkably, all this fits together in a smooth presentation of the most massive, complex war ever fought. Though the game falls short in individual areas, it comes together as a whole - unlike its predecessor. It's easy to plan offensives while paused - set day, night or 24-hour air strikes (of various kinds) against your targets for any number of days or weeks. Plan air superiority over the area and then set a time of attack for your armies. Always aim to cut off retreat, so you can wipe your foe out completely and not have to fight him again when he regroups.
While the battle ensues, you may have to establish protective fighter patrols over your industrial areas, and decide your next research project. Do you get the better tanks, aim for a new land doctrine, or try developing technology that may lead to unlocking one of the secret weapons? Once you decide that, it's time to select a research team - pick the most appropriate for the job. Finally, you have to make sure your researchers are getting the funding they need, so your economy has to produce consumer goods and you can't blow that cash on diplomatic bungling.
Best of all, while Hearts of Iron 2 isn't a simple or easy game by any stretch, it's not difficult to wrap your head around the concepts or navigate the various menus and map modes. Despite the added depth, it's easier to learn than the original.