@matrix @zemichi
I guess it all is just depending on your definition. In my opinion, it is just about the universality of their abilities, and incompatibility of them with the backstory.
Why so complex definition? Because there are some characters, that at least for me embody the concept, even if well written.
The Doctor: This mary sue is something truly special. Nit only does he have every important information in the universe, but when that is not enough, the universe itself will break just so he can win... (I am not making this up. This was in the last two episodes before I just gave up)
So no, Superman is not a mary sue IMO, because he has many equals, and won't win everything... Contrary to the Doctor at least.