Well let's see, I can tell you what's gone in so far: your mom.
But that's no secret, I already gave that away awhile back. So let me think.
Chapters one through H are keeping mostly the same organization.
Chapter 1: A new section called "Spirits of the Rules" tells about Almighty Bennies and Unmighty Pennies. I'm also thinking about adding a bit about VicLUG's all-WISG game.
Chapter 2: An illustrated guide about making stat cards.
Chapter 3: Some minor numbers tweaks, and Tools are now separated out from other random objects into their own category.
Chapter 4: nothing fancy.
Chapter 5: some upcoming cleanup on Close Combat to make it easier to understand; there'll be less distinction between attacks and counters. Charge attacks are now only described for minifigs; Charges for larger creations are moved to a later chapter.
Chapter 6: may be where I put the Cannon Fodder, not sure yet. Also I may include Manly Santa as another Hero example.
Chapter H: simplification of Charge and Joust rules.
Everything past this point is where the big changes are; I've busted them out into "Book 2: MOC Combat." The chapter breakdown is changing back to be a lot more like BW2001.
MOC Combat: Explanation of how Creations work, and how to use all the new dice besides d6es and d10s.
Ch 7: Structures. A new section called "Field Construction" where the Mechanik is re-introduced; Construction Actions; Patch Repairs, Disassembly.
Ch 8: Weaponry. New weapon types: Blast guns (like shotguns), Automatic Weapons, Flamethrowers. Firing Arc measurement using fingers. I've been hung up on the Flamethrower section for awhile trying to get fire to work right, but otherwise these are all done. Next up is a weapon type for Environmental Hazards (mines, barbed wire, etc) and reintroducing the Scout who can circumvent them; introducing the Gunner, a new unit who specializes in firing larger high-Use weapons, and the Heavy, who can handle vehicle-sized weapons.
Ch 9 (Upcoming): Vehicles. All the old propulsion stuff and the Pilot. I'm still deciding on whether some of the systems from the old NanoFleet prototypes will work their way in here, most likely the battlestations rules if anything at all, possibly some of the vector propulsion stuff in a very limited form for naval engagements.
Ch 10 (Upcoming): Creatures. Rules for limbs and creature- and mech-specific abilities. Creature injuries and re-introducing the Medik. Supernatural Powers make their return with a much more straightforward system than in 2001. I may reintroduce the Ninja as a SP example, since I want to put him in the book and can't figure out where else he'll go. Dimmies, Jaw-Jaws, Skellies, and Zombies also go here.
Chapter 0937 (Upcoming): Squids. Rules for BrikThulhu.
Chapter 11 (Upcoming): Squads. I want to move squids and squads back to Book 1 right after chapter H, but it depends on the order in which I need to introduce some of the mechanics. I want to simplify the hell out of this chapter, I rushed it last time around and it came out way too complicated.
Lastly I want to add a bunch more world fluff all over the place like what's been developing in the wik: peach suppression, protofigs, the Deadly Spacemen, Yellow Castle and the Vladtron Rebellions, the Farce, etc. I'm working off and on on the new Introduction chapter which has some bits about clone brands, minifig customization, and the Valkyrie painting with the big Lego boobs, so I can tell people their parents are idiots.
So, nothing earthshaking, it's mostly just restoring a bunch of the better parts of 2001 but in more 2005ey versions. Is there anything I'm forgetting?