obi_wans_beard
Is fulfilling his destiny
One of the problems was that his upper face was too dark, it was severly encased in shadow thanks to the hood. In Empire we can see his eyes and forehead a lot more clearly under the cloak, but in Sith it was very dark and his eyes looked very strange indeed.
I thought that his face reverting to the ROTJ look was because he was using so much power he is unable to disguise his appearance. I think this is backed up by his hands going pasty and his finger nails going sharp and a dirty brown colour (they were all white in Jedi!) this couldn't have been caused by the sith lightning so it just must be his true appearance.
George really should have had Palpatine say a line of dialogue so clarify this to the audience because it is pretty confusing. Something along the lines of "So now you have seen my true face and the power of the dark side!!!!"