Ok, first of all the Premise.
A virus begins rewriting human DNA, so some humans end up with neat super powers, some end up horribly mutated, some die upon being infected and some appear to be immune.
This premise was handed to a whole bunch of different SF authors who each wrote their own characters and short stories - which they then traded to each other for tweaking and Mr Martin turned it into a semi-cohesive whole.
So, each story is different and has it's own flavour. Some dark, some light, some positive, some negative.