Is there a reason for items to disappear when an unlooted or not-sacrificed corpse decays? On more than one occassion, I've beaten a tough mob for gear only to lose out because I was blind and couldn't sacrifice or loot the corpse before it crumbled.

It's that way just to piss you off! Okay, it's actually probably that way just because it always has been that way. I wouldn't personally have a problem with making mob gear end up on the ground when the corpse crumbles. Alright, seriously, it's that way just to piss you off. :)

Aarn?

Your in a dark room trying to get a glowing item. We'll say the elf in the goblin village. When you go in, the room is lit up, because the elf is wearing the amulet, but when you kill him, it becomes dark, making you unable to get the amulet from his corpse.

Actually, I believe there are some mobs that were written with the intention of making their things hard to get by having them blind people, an example would be getting keys from certain things in the Octagonal Tower, and in those cases, it might go against the origional intention of the author to change it, but that, of course, is not up to me.