Even though it had an insanely steep learning curve, I enjoyed the trading and extremely detailed tech trees of HoI I
I cannot remember what I enjoyed about HoI II except the individual tech teams, it added flavor, I played the hell out of that game and had all the expansions.
I really like the extra provinces in HoI III, it makes so you aren't constantly going through the exact same paths when attacking. I enjoy the HQs and leaders and the variety of units you want and can build (there a number of mods out there that make this even better)
So I am hoping for a better trading system, a better supply system (come on supply depots!), the detailed flexibility of building and controlling units, country specific tech trees and units, better hover over descriptions of game play (this has been alluded to in some of the dev posts) and of course, and less flaky AI (defend against invasions as the UK, why would we do that!?).
A lot of the new stuff looks really good but I am not sure about the air system as the regions seem way too large and you cannot seem to concentrate air forces in specific provinces as you used to. I know it is just a general overview so far but *shrug*
Paradox has always created games that have given me thousands of hours of enjoyment so my hopes are high.