Re: Kryptonite in first film... Extent of use?
Kryptonite should only be used when nothing else can present a physical threat to Superman. Ultimately, it's only a plot device as without it we have a Superman who's basically invincible against 99% of his enemies and threats, and the lack of perceived threat to our protagonist removes any tension or buildup from the film.
Superman vs Zod?
Superman vs Doomsday?
Superman vs Brainiac?
All very evenly matched, so no need for Kryptonite.
Luthor presents more of an intellectual threat, and Metallo is not as powerful phsyically so both rely on Kryponite at times to strengthen their position.
I personally wouldn't like to see it getting overused in the film. Let see the writers actually engage their brains and come up with something which can threaten Superman and isn't green or radioactive.