
The main difference I noticed was that when you press start, it gives you a course select screen:

Normal: Exactly what is sounds like, no differences.
Harder: Exactly what it sounds like, makes the game noticeably harder, I haven't tested to see if this is affected by the DIP switches for difficulty
Extended: Gives you the full game, even if you continue.
Special: Makes the game EXTREMELY hard, moreso than harder. Haven't tested to see if it's affected by DIP switches.
Direct: stage select mode, change stage with left and right.
Within the game, you can pause the game by pressing start and slow it by holding start. Unlike later games, you cannot access special functions by pressing C while the game is paused. More testing is needed to determine everything.