Bonus unlockables
Regardless of difficulty setting when you beat the game you unlock various things

Unlock 2 Batmobile Missions:
Beat the game once

Unlock ALL Movies and Interviews:
Beat the game once

Unlock Everything in Gallery of Fear:
Beat the game once


Unlock Batsuits
The following old Batman suits are unlocked at the same time when you beat the game regardless of difficulty setting.

Unlock Caped Crusader:
Suit Complete the game on any difficulty

Unlock Dark Knight Suit:
Complete the game on any difficulty

Unlock World's Greatest Detective Suit:
Complete the game on any difficulty