Batman always has a plan and a gadget at the ready for just about any situation he gets into, no explanation about how he dreamt up the scenario or developed it, he just has.
Superman's power has pretty much always been: he's Superman therefore he will have the strength, speed, courage and resolve to save the day.
I am only using these two, as an example that even characters as old as them, along with Diana and everybody else are always vague and whenever someone sets an actual limit (like Marvel's 100 ton scale for strength), people who come after will either ignore it or come up with a solution to break it. And that's just how it's always been.
By classical heroics I mean the likes done by Perseus, Odysseus and Heracles.