Manly things.
Common complaints such as too short or bad ai can be diminished via higher difficulty settings and then there are some games that REALLY come alive with the higher difficulty settings, such as Halo, Hitman, Splinter Cell (original trilogy) and Timesplitters 1 & 2.
Games that are not very dynamic (or basically played on autopilot) like COD, Uncharted or Battlefield games makes no difference whatever the difficulty is. There is no need to play on anything above easy on such games and the difficulty does not change the gameplay by a single ounce.








