It's all about the old-school games. There's something inherently disturbing about the sparce black backgrounded graphics, the electronic noises and music and the disturbing monsters that don't look quite right hehe
Also, a lot of them relyed on clever writing to scare you.
"Kneeling down next to the fountain, you drink a
handful of the acidic water. You can't scream because
you no longer have a throat, let alone a larnyx!!"
How scary and harsh is that? hehe
try Shadowgate on the NES, in the dark, late at night, with headphones. (use an emulator) It's an ancient game but you will get creeped out at least once. (Yes, I went back later and confronted my fear of that game)
Oh and the bit in Monkey Island? Here's the answer
http://www.worldofmi.com/imageviewer.php?id=19 and it moves "Aww, he likes you!"