It's not as simple as just beating a game under certain amount of time.

Adding actual difficulty to a game means more then just extended gameplay time. It adds a sense of desiring to accomplish something, and then gaining that sense of accomplishment. It also sometimes even replay value if you so desire to go back to a game that's been difficult for you in the past (it all depends on the person). It gives players a meaningful challenge to complete and a motive to complete that challenge -- it adds depth; instead of the game just basically walking you right through the game while holding your hand. In most cases it also gives diversity to the gameplay as well.

It can never actually hurt the game just to add a difficulty level. Miyamoto seems to think so otherwise though. . .


EDIT: At the top of my head:

- Resistance: Fall of Man
- Ninja Gaiden Sigma
- Tom Clancy's Rainbow Six Vegas
- The Elder Scrolls IV: Oblivion

(Although to be fair, those last two are on the 360 as well.)

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