Good software design nicely summarized in 4 simple (but deep) points

1. Create crisp and resilient abstractions.
2. Maintain a good separation of concerns.
3. Create a balanced distribution of responsibilities.
4. Focus on simplicity.

taken from: http://www.cio.com/article/373215/_Things_Grady_Booch_Has_Learned_About_Complex_Software_Systems?page=1