some of this is already mentioned, but here's my two cents:
- Game turns can be played in single turns, with movement, fire & melee all taking place in a single segment - with the defender taking "opportunity fire" as the offensive player is moving. This gives the game a bit more of a "real time" feel without requiring the players to both be on-line at the same time. It is still possible to play in phases too, if you select that option. The single turn mode radically reduces the amount of file transfers needed to play a PBEM game as well.
- Losses are now handled in single man increments rather than in 25 man segments.
- Fatigue is now handled in single points too, rather than huge blocks - though some things can really make the fatigue rack up, such as loosing a melee.
- Campaigns. There is now a feature which allows you to play through a series of battles, making a decision before each which will directly impact the positioning of your forces for that battle. Losses are also carried over from battle to battle in most cases, stragglers can return to units, and if a short amount of time passes in between actions fatigue will also carry over.
- Maps...the format of maps have changed from a single bitmap image to a "tiled" approach. This allows MUCH bigger maps to be used, and also allows many more maps to be included with each game.
- Content - tieing into the map comment above, there is also a significantly larger amount of content included with each title now compared to the Battleground games. For example Campaign Gettysburg includes more scenarios & maps then all five of the BG ACW games combined - really, not joking, its a huge title covering the opening actions at Brandy Station through Gettysburg and then the retreat. You get a good portion of Northern Virginia mapped, Maryland & southern Pennsylvania.
- There are many more engine enhancements as well, which you can read about by checking the "Changes.txt" file from one of the games on our site. Here's the one for Corinth, which was the first title released with HPS -
http://www.hpssims.com/Pages/updates/up_...th_108.txt - these games are still supported as well, with new content being released for the entire series as new games are published, for free.
http://www.hpssims.com/Pages/updates/up_...ivWar.html
The series also is well on its way to cover the entire scope of the Civil War, where the BG series never will. So, all in all it is a far better value, and will continue to be, over the older games.