The doors on both the Glory and the Conquest were made of "formica- like" material.(actually the same pattern as my counter-top at home) Regular scotch tape worked well and was easily removed with out any residue.
We decorate our doors so that we can easily identify our rooms and those of our traveling companions. I know we could use the numbers, but this is more fun. Last cruise we were celebrating anniversaries and birthdays while on board so we used birthday and anniversary decorations. Our first cruise we used "Sponge Bob" and our friends (who happen to play in a band) used a "Rock n Roll" theme.
We have not seen many doors decorated, but we rock to the beat of a different drummer anyway and like to be a little different.