I think it's the ambient music that's scary
and horrific mutants throwing themselves at you.
( Major Turd for tonight only, Sat 13 Jun 2009, 17:26, archived)

F.E.A.R is creepy as well
so any game that's an acronym it seems.
( Undulating Tentacles of Love getting fun down to acceptable levels, Sat 13 Jun 2009, 17:42, archived)