It uses the stats for monsters and characters mostly unchanged. If you know the combat rules for D&D, you're already 90 percent familiar with how Battlesystem works.As I wrote in 2011 for section on conversions (p. 10), this was also one of the guiding principles for Book of War (prior games from TSR all had quite different mechanics & statistics; see all of Chainmail, Swords & Spells, and Battlesystem; contrast with the Book of War core rules here).
The big changes in the rules focus on scale. Large battles naturally take more time than single combat, so a round of combat in a Battlesystem mass battle takes 1 minute. Battlesystem uses a combat grid divided into squares measuring 20 feet on each side, and scales up the number of creatures a single miniature represents.Note that Book of War is also at 1" = 20 feet, whereas all the earlier games from TSR were at 1" = 30 feet (as above, see assessment that led to the Book of War scale here).
If a stand attacks a solo creature, the stand takes one attack for each creature in it. A solo can avoid such potentially devastating attacks by joining up with an adjacent friendly stand, relying on the creatures in that stand to protect it. The enemy stand can still attack the solo, but it makes only one attack.This "devastating attacks" statistical observation re: solo vs. mass was incorporated into Book of War, and in some circles was the most controversial part of the game; the prior Battlesystem from TSR had explicitly inflated solo stats to make them super-durable on the battlefield (while fantasy Chainmail was man-to-man only, and the implications in Swords & Spells were obscured; more discussion here).
Of course, I agree with Mearls that this kind of approach makes for a much more satisfying add-on game to D&D, and I'm glad to read that Wizards have pivoted away from the 3.5/4E approach in the last several years. Coincidentally, I've had an unusually large number of friends staying over at our place for the past month, and we've actually been playing a lot of Book of War lately. It's great to see how excited folks are by the game when first introduced to it, and then perhaps to the closely associated Original D&D game as well. I'll probably have some battle reports coming up from those games -- and if you want to check it out, of course, Book of War is available right now in the sidebar and here at Lulu.