Everything Comes At A Price

Statement: In a good video game, all strategic options must have some sort of cost associated with them.

In melee-oriented action games, weapons have unlimited ammo, but enemies are more aggressive and there is an inherent risk in engaging in close-quarters combat.

In Contra eaach weapon functions differently and has better applications in certain situations. Or, at least, they would if not for the sheer overpowering fury of the spread shot.

In roleplaying games, powerful attacks might have a low hit chance, a low attack speed, or a huge mana cost involved.

In DOOM, every weapon uses a specific kind of ammo and fires shots in a unique manner, giving each weapon a proper situational advantage. The pistol can clip easy guys, the shotgun can handle just about anything with some patience, and the rocket launcher is suicide in close quarters. The BFG-9000 is powerful but has a huge start-up time and ammo cost associated with it.

It's about variety. Don't make a hundred things that are the same. Make ten things that are wildly different but still somehow balanced. It's arguably fun.