For a while there it seemed like Superman's powers were nearly limitless as he literally moved planets and blew out stars. As said above Byrne sort of dialed back the powers a bit to Superman not being able to survive in space longer than he could hold his breath, which still was a long time.
But "realistically" it would seem that on Earth, unless near some form of Kryptonite, his powers have no bounds. I mean he can take a hunk of coal and compress it into a diamond, something it takes
nature eons to accomplish. How he manages to shake someone's hand without crushing their bones into a blackhole is a power within itself. The only "limit" to his powers is him not being able to see through lead with his "X-Ray" vision. (Which under logical interpretation involves no X-Rays, but he simply has some-sort-of "see-through" vision.)
Now there are other things he is weak to, magic being one of the things and I believe to other things that can effect his mind like hypnosis.
But he can lift anything on the planet he needs to lift, he can run at relativistic speeds, his heat-vision can melt everything, his super-breath can freeze anything, and he can compress coal into a diamond within seconds. So he's got some pretty strong powers in his corner. At the same time, I've never bought into the idea that Superman is so over-powered as that it's hard to create a foe for him to fight, because you can with foes that can out-think Superman (again, something that's probably hard to do) or just in how he handles the situations he's in to do good for the people of Metropolis.
Super Hearing - hearing over interstellar distances?
This is doubtful, hearing over interstellar distances isn't likely as sound doesn't travel through space.
Maybe if Superman's hearing is an "active" process (his ears emit something that picks up sounds at their point of creation rather than waiting for the sound to reach them, this is likely as he's able to react to a gun shot before the super-sonic bullet reaches its target) but even then I doubt he can "hear" over interstellar distances, at least of the current writers want to keep Superman's power range within some form of reason and just keep him Earth-bound, not needing to hear into space. Superman: Returns did suggest he can hear from space, presumably everything on the planet, which again assumes active hearing.
I think it's mostly been suggested Superman can "tune" his hearing to filter out what he doesn't want to hear and only hear what he DOES want to hear.