Model Display
By completing the game with a ranking General, you'll open up this new feature on the options screen. It allows you to view every single aircraft in the game.
Secret Missions
Upon accessing the Free Mission mode, if you play through the familiar missions again and complete them all on Easy, normal and Hard modes, you'll be rewarded by two bonus missions that ain't in the normal game. These can only be accessed in the Free Mission mode.
Alternate Mission Mzp View
Highlight the mission option on the mission select screen and press select. Three new viewing angles can now be chosen.