From Diablo Wiki
Goldilocks is a term used for a game feature or system that is well-balanced and functional, This is difficulty to achieve in video game design, and it's always a judgment call if a feature is just right, or not.
The term refers to the classic fairy tale, where Goldilocks enters the home of the Three Bears and finds the porridge too hot or too cold, and the beds too hard or too soft. The name has come into common usage to refer to something that's "just right" for example, astronomers and exo-biologists refer to a planet that's in the right temperature range for liquid water as being in the "goldilocks zone" for potentially supporting life.
- See more Diablo 3 Reference terms.