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 to provide examples, common mistakes, or anything that improves their answer!

In this question the solution is part if the puzzle!