Here are some things that can contribute to a better gaming experience:
Skippable cut scenes
Multiple skill levels (and, if possible, independent skill levels for player 1 and player 2)
Multi-button controller support
Auto-check additional diskette drives
Rigorous play-testing (by people other than the programmer)
Control customization (ability to assign controls to keys and buttons)
Extensive audio options
Option to quit/exit game at any time
Save feature (if applicable)
High score table (if applicable)
Pause feature
Items that aren't essential but are nice to include:
Save states
Level editor
Options menu (vary number of attempts, enemy shot speed/frequency, time limits, etc...)