When making a new puzzle, the average person is usually faced with a hard problem of guaranteeing quality.

I want to know some key points and ideas, for beginners and experts alike, to keep in mind when creating their puzzle, because a good puzzle must be well-polished. What are some features of good puzzles that apply broadly to a wide range of types of puzzles? I would imagine careful management of difficulty - something like the requirement of play-testing - would be an aspect to consider, for example.

Answerers are encouraged (but not required) to provide examples of features, common mistakes, and ways to improve a puzzle.