When making a new puzzle, the average person usually has a terrible problem when faced with the issue of guaranteeing quality.

I want to know what some key points and ideas for, beginners and experts alike, to keep in mine during the creation of their puzzle; because like any product or idea, a puzzle needs to be well polished to make it look good. What are characteristics, in general, regardless of "puzzle-type" which are common to good 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, common mistakes, or anything that improves their answer!

In this question the solution is part if the puzzle!

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.