Concept: Philosophical Zombie Apocalypse
Description: A contagious disease/curse destroys the consciousness of its victims, leaving them as thoughtless husks that act exactly like they already did. The invisible pandemic sweeps through civilization, thousands "dying" without anyone even knowing. Left unhindered, sentience itself would be erased, leaving everything as fleshy automatons that act in perfect mimicry of sentience.
Source: An original idea as far as I know.
I think we're living that today. Ever see people focused on their phones running into all kinds of crap due to a lack of awareness?
The situation would be the exact same in Pzombieland and regular world, that's the entire point of the thought experiment. Basically, you as viewed from the outside would remain perfectly identical, whereas your consciousness, your 'self', would be annihilated.
Concept: Non-predefined universal laws (a bit hard to express as a title)
Description: As described in the Religion thread - instead of a creator setting up all the variables and laws on day zero of Creation, the parameters of the world were left undeclared, since the creator couldn't be arsed to pick anything in particular.
Sentience was created to outsource coming up with how the world works. The first creatures formed their ideas of the world around them, and the creator picked whichever submissions he liked and made them universal laws. Then came next creatures, then humans, each generation solidifying a part of their ideas of the world outside them into the actual shape thereof.
Essentially a half-brother to the Consensus Reality concept of things like White Wolf's Mage tabletops, except here, instead of being mutable at a whim of the general population, the laws are set once an idea gets metaphysically stamped. Such a world's magicians would be essentially the cosmic lawyers, finding undefined areas of reality and moulding them in a way that allows them to manipulate the resulting reality. The more secretive sorcerers of the past would create the classic ritualistic magic, as it would limit the abilities they introduced to just themselves and their disciples, whereas those who didn't care much for being privileged would make it into a quasi-science, or open-source magic if you prefer.