Hearts of Iron is a grand strategy that allows you to take control of pretty much any country during the period of World War II. This can be anything from the major powers (Germany, England, USSR, America and Japan) to the many minors (Albania, Cuba, Mongolia, and dozens more).
The game allows you to control the politics, economy, scienctific research and military of your chosen country. You are then allowed to follow whatever path you want, creating your own alliance, or joining the Axis, Allies or Comintern. You can achieve anything given you try hard enough.
HoI3 introduces many new features, including a revitalised AI, which can be told to take over aspects of your country, to allow you to focus on only specific areas. The developers have been releasing alot of information on the game through Developers Diaries on their forums, as well as several previews written by gamers that took part in beta/press releases.
Paradox Interactive also supports their games indefinitely, with patches released quite often, even for games released 4+ years ago. Bugs are also usually ironed out quickly.
Fans of the Europa Universalis series, Victoria and Crusader Kings should definitely keep an eye on this game.
Those who enjoyed Civilisation and the campaign map of the Total War series should download the demo when released and see if this is their type of game.
It is important to note that many previews (while positive) are quite uninformed, and seem to give good ratings because they don't really know what this type of game is. The Grand Strategy genre is a bit of a niche, and many people are quite intimidated when they first learn to play these games.