Paradox Entertainment has made some of the best open-ended historical strategy games ever. If you wanted to re-write world history, the Europa Universalis games were your ticket. So, Hearts of Iron, Paradox's World War 2 grand strategy title, always seemed a little out of place. Events didn't always turn out historically, but you could pretty much count on Austria succumbing to German political pressure, the Czechs being forced to relinquish the Sudetenland and Poland being first to be attacked. Although the outcome wasn't always certain, the major events were pretty much scripted.
Hearts of Iron 2 seeks to remedy much of this. We've spent about a week with a preview build of the game and while scripted events do exist, they're not the all-powerful determinants they were in Hearts of Iron. Thus, while Germany can get the Sudetenland and Austria, events are much more open-ended than before.
Paradox has also fixed just about everything else that was a problem in Hearts of Iron. The gigantic, unwieldly armies? Gone. An obtuse and unwieldly upgrade/reinforcement interface? That's been replaced as well. Normally, "Paradox" and "user friendly" aren't two terms we use in the same sentence but the development team has really made the game a whole lot easier to work with.
Many other issues have been fixed as well. Aircraft no longer simply keep flying missions over and over until told to stop by a player. They have a timer now - the player can tell them when to begin, when to stop, whether to fly night or day missions (or both) and at what strength level to stop at. Thus, you can tell your bomber squadron to run interdiction attacks against an enemy armored division for a week, but only at night and only as long as they have at least 20% combat strength.
Other changes have been made to the combat system. Armies no longer engage only once they've crossed provinces. From the moment an order is given to attack an adjacent enemy provinces, combat ensues. There are several stages to combat, with percentage chances to do various special effects like Encirclement, Ambush and Breakthrough. These chances, and of course the unit's overall statistics, are enhanced by research.
The research tree has been streamlined a great deal. There are nine categories to research and these overlap each other far less than in the original Hearts of Iron. A player simply researches "Advanced Light Tanks", rather than having to research first a prototype tank, then armament like a 40mm gun, and finally the tank itself. This has less detail and may turn off the grognards, but it also makes the game easier to deal with.
Another thing to know when researching is that there are now dedicated research teams. Germany, for example, has Krupp, Rheinmetall, Mauser Work, General Guderian, and Dr. von Braun among others, doing this work. All these researchers have specialties they can apply to various aspects of a project. Thus, it's better to keep Focke-Wulfe Flugzeugbau working on aircraft while Heinz Guderian researches your military doctrines. Only up to five teams can be operational at a time, though this is the maximum in-game - some nations have less than this.