It HAS been done to death, and I'll answer your question before people come on to tell you your mistake in a less than nice fashion.
There are two possibilities, and I prefer the first...
1) The intensity of Palpatine's own power came back on him and melted his face, the official answer from Starwars.com and the Visual Dictionary...
2) Palpatine's 'Chancellor' face was a form of Sith Alchemy that could be used to change facial features, which seems somewhat absurd but at the same time could make sense, if there was no official answer, like #1
The reason Luke isn't 'melted' is because a) it is not his own power, and b) Palpatine isn't trying to kill him...yet; he's torturing him, and that final blast is when he starts to kill him, when Vader picks him up...