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**!*