This is another genre I think would be really cool, and very easy to do with Swords & Wizardry. There are a ton of monsters already on the books, many of which would fit fairly well as mutations from magical fallout.
For this setting, I don't think there would be much left as far as wizard labs or libraries. If one is found, it will likely be hidden and very well guarded, with both monsters and traps. Fighters and rogues would be supreme, as mages would have a really hard time finding a teacher, researching spells, or creating magic items. I'm sure there would be at least one heavily fortified town with a mage either ruling it outright or from behind the scenes, though.
Clerics could go two ways: either they are the common spellcasters with a lot of power, or the gods have left and clerics no longer exist as a class. I prefer them not being around, as it makes the setting more desperate.
Common problems faced by post-apocalyptic fantasy characters:
Food and water.
Poisonous areas caused by magic. Maybe people just get sick there, or maybe they are magically mutated or slain.
Undead. There will be lots of these. The mass deaths of the apocalypse combined with clerics losing their powers would just make their populations increase. I would keep this way below zombie movie levels though. I want undead to be a threat, but not the only major problem.
Creatures such as gibbering mouthers and beholders would roam freely, as they are either unaffected by the magical disaster or even strengthened by it.
More beastmen and animal mish-mash monsters to represent all the mutants caused by the magical blast.
Magical items would be very rare, and extremely valuable. Spellbooks would be the equivalent of an artifact.
Potentially a magic-immune race could develop, that gets inherent magic resistance.
Ruins would include entire cities that might be empty, or could be full of gangs of people who used to live there but now fight each other and might view travelling adventurers as food or pawns to use against the gang up the street.
I could go on. There's just a ton of really fun stuff that could be done in this type of setting. I believe there is a OGL game that covers this genre, though I haven't seen it.