Ive always hated it when a game gives you weapons/units/powers at the beginning that have no use later in the game. For example in Warcraft 3, you have no real reason to create footmen if you have access to knights, thereby making footmen useless.

I love the idea of any weapon being used against every enemy. We're not asking to kill a fighter jet with a molotov coktail easily, but having a small chance for the jet to be ambushed while it is flying low and having the coktail liquid seep into the cockpit. Things like this definitly give more "subtance" to every object, instead of fodder to replace with better weapons.