Very cool thread!
Mine might be stretching a little....
-- 5 out of 6 movies had Anakin OR Darth Vader, but ROTS had both.
and
-- 5 out of 6 had at least two guys named Darth, but ANH had only the one
TPM -- Maul and Sidious
AOTC -- Sidious and Tyranus
ROTS -- Sidious, Tyranus, Vader
ANH -- Vader only
ESB -- Vader and Sidious
ROTJ -- Vader and Sidious
(or conversely, only ROTS had three and the rest had two or less)