Chapter 4: Alignment
Oddly enough, there are no optional rules in this chapter, but the alignment change section is different from other editions(I think) to the point where it needs to be mentioned: The DM determines if the change was voluntary/involuntary and whether or not the change is for the betterment of the game.
In the case of a voluntary change for the betterment of the game: No penalty.
In the case of a voluntary change not for the betterment of the game: Instead of losing a level, the character is required to earn twice as many experience points when qualifying to earn his next level.
In the case of an involuntary change that the player has no intention of keeping: The PC gains NO xp until he undoes the change
In the case of an involuntary change that the player intends to keep: The moment the character decides to take this route, it is then considered a voluntary change not for the betterment of the game.
Chapter 5: Proficiencies
The entire chapter is one giant optional rule
Chapter 6: Equipment
PCs roll for cash, and buy everything one piece at a time. Encumbrance is 100% optional. In the DMG it states that PCs must spend a certain amount of cash each game month to survive. I know I've never actually used these rules before as I always preferred the PCs to go ahead and spend as they had the cash/needed for 1 night at an inn here, 1 night at an inn there, and have to buy/catch/grow/find their own meals. It's also here in the equipment chapter of the DMG that weapons of quality(Masterwork weapons for those more familiar with d20) are discussed. Prices for these are not static, instead costing from x5 to x20 the base cost of the weapon. Ornamental weapons are also discussed here, and the suggested price for them is x10 the base cost of the item.
Beyond this is the mention of item savings throws and the amount(and type) of damage needed to break certain objects(chairs, rope, wooden doors, etc)
Tomorrow we'll at least START talking about Chapter 7: Magic and possibly Chapter 8: Experience