Someone please use this idea, if you're running a 4e D&D game. I suppose this could be used in any game, really, but all of the juicy mechanical bits seemed right for 4e.
Take any outdoor scrap, and throw in a terrific lightning storm!
Jagged bolts of lightning crash furiously all around and amidst the fight. Each round, roll 1d6 each round for any character with significant metal (armor, weapon, etc). On a 1, let them make a save of some kind for half damage against a lightning strike. Use the Damage by Level thingy to figure out how much damage to dish out, but it should be on the high side, probably. Goes for PCs and monsters, of course. Keep in mind that if armorclad PCs are fighting some kind of non-armory opponents, it'll put the PCs at a more significant disadvantage than if they were fighting chainmail orcs or something. For anything not in armor, make it a 1 in 10 chance of a lightning strike. Oh, and let anyone hit, and anyone adjacent be stunned or dazed for a round.
For added fun, throw in some rain-slick, anyone moving more than 10 feet makes a save or ends up prone.