as far as I can tell, pyrovision selectively replaces some textures and models and various other stuff to make them match the "fantasy land" aesthetic in the pyro point of view sequence in the meet the pyro video.
It only appears to occur on certain maps - I've seen talk of editing a map's .cfg to enable pyrovision, but I wonder if that is all there is to it, or do we have the option to do more stuff on the hammer end of the equation to control what it will look like in pyrovision?
As an example: one of the maps supposedly had christmas-type lights added to a power line or some other sort of wire with pyro vision enabled. This seems like it would be something you'd have to design in - but that's all speculation on my part.