Software ideally matches a person’s mental models. This means that the software works in the way the person expects it works.
This can be improved in some cases with familiar interaction metaphors. But metaphors have limits, so this is not always the right approach.
Any apparently physics-driven simulations in the software should match the person’s expectations.
Where appropriate, direct manipulation and other natural interaction methods should be used to encourage quick understanding.
Actions and animations should be interruptible where possible. People shouldn’t have to wait for the software to show off.
Even if a person has spent a long time away from the software, it should be easy to pick back up.