There was a cool AddOn, called BloodyScreen
http://www.curse.com/downloads/details/4401/ , that would splash blood across your screen whenever you would crit in combat (it looked somewhat like Gears of War style blood). Unfortunately, it hasn't been updated for 3.0. I know nothing of coding, so I wouldn't know if it would be hard to update?
I think it is a pretty cool AddOn, one of the few that just make the game more fun, rather than crowding your screen with more numbers and stats. If anyone would be able to fix this for current WoW patches, or tell me how to, it would be really cool, as the old author has abandoned it completely (as far as I can tell).
Thanks in advance for any help you can give me, have a good night.