It would be cool to see something like this implemented as it would encourage skilling in different weapons types. There are the general spell resistances so why not melee?
One example I can give (from a DnD PoV) is skeletons. Skeletons in DnD have resistance to slashing but take more damage from blunt weapons. Obviously fleshy creatures this wouldn't apply so much but this would give a unique twist on certain mobs. Earth elementals... Metal type creatures(Golems?)... Dragons... etc. etc.
Thoughts?