Most randomly dropped items have a higher rate. It's generally between 50% and 100% higher. Some of the boss bags give two drops, as well. The Wall of Flesh bag, for instance, always drops one emblem and one weapon (plus the expert item itself). Coin drops are roughly doubled as well.

Shackles should be dropping a lot more, therefore, and while I've gotten loads of other random items, I really haven't gotten many shackles or zombie arms, which is unusual. I think it's possible that the weapon-wielding zombies don't drop normal zombie loot, which would account for that.

While I like Expert mode on the whole, I wish there was a different implementation of difficulty rather than just doubling everything. Certain high-damage monsters with difficult-to-avoid attacks (turtles, rune wizard) are simply brutal in expert without being satisfying.