sith lightning

And along with what BakaNeko is saying about the lighting coursing all over Luke's body and not just his face, let's not forget that Luke was a very strong Jedi. Remember, Anakin got a blast of lighting directed at him from Dooku in Episode 2. It didn't disfigure him, it just immobolized him for a short period. Palpatine was blasting a huge amount at Mace because he was trying to kill him, so when that amount of evilness was transferred back to him, from Mace's lightsaber, his face caught the full brunt of it.