Any game that makes you choose between music or sound needed to be fixed to support both :/
Turrican 2: why that ear wax-liquifying noise when you are hit???
Jim Power: a health bar / hearts would have made the game a lot more playable.
Armourgeddon: It seriously needed a tutorial. To this day I have no clue what to do.
Toki: a health bar is not needed in this game, but slightly less vindictive collision detection would have been nice.
9 lives: this game would have been playable if it wasn't so jerky and didn't have such a weird timed jumping mechanism.
Cannon Fodder: would have been nice if people waiting in line would get some kind of training based on the progress you make in-game, so you wouldn't have to play with complete rookies if your veteran guys would die in the field. Its the Project-X principle - if you die in later missions, you might as well load back a game / start over.
Silkworm: more variation in the bosses would have made this great game epic.
Yeah, you can go on and on and on...