I'd say: A good abstraction is a formal and abstract statement about the nature of the underlying (sub)system. It is self consistent and allows an easier way to reason and interact with greater complexity underneath.

And the more you see an abstraction as a "wad of API calls and functionality", the more leaky the story gets.

Some systems are notoriously difficult to get this statement right because they are volatile and prone to changes. But I think as in live or in academia, our aspirations should be to strive for a better abstract understanding and keep building better abstractions.