Old people who remember the 1980's "Handbook of the Marvel Universe" will remember that Marvel once actually quantified their character's powers. Cap can deadlift 2 tons, Spideyl 10 tons, Thor 100 tons.
Unfortunately, quantifying powers limits them so they can't be altered to fit the plot. Now Marvel uses a non-quantified 7-point graph. 1 is "under human normal," 1 is "human normal," 7 is "hell if I know, but it's a lot." Cap is now 3 for strength, Spidey a 4. Note this makes Cap - whose power levels are merely supposed to be "peak human perfection" equal to Spidey, who is supposed to be "Superhuman." I guess that's as close as we get?
Superman is NOT merely vulnerable to "kryptonite, period." Supes is vulnerable to magic, psychic powers and, of course, fluorescing helium. Wonder Woman at times carries a magic sword enchanted by Ares, and it WILL cut him. Diana could carve Kal-El up, if she really, REALLY wanted to.
Maybe. My memories of DC power levels basically ends between Zero Hour and Final Crisis. I have no idea how power levels got rejiggered after New 52.
Either way, it's "fit the plot." Why did it take Superman three minutes to catch and stop a plane in "Superman Returns," but less than a minute to lift a literal island of Kryptonite into orbit and hurl it into the sun AFTER being stabbed with Kryptonite that had broken off inside him? Lifting the island wasn't cross-cut with shakey-cam of Lois in the plane thinking she was going to die. Speed of plot.