Quote:
Next up I assembled the puzzles in-game and turned them loose on our testers. I quickly discovered that what seems easy to me after working on the game full-time for nine months isn't necessarily as easy for gamers who haven't spent that type of time with the product. Test, test test. Who knew? There's a fine line between challenging and "Nuts to this" with gamers.

If you have a few minutes and you find puzzle design interesting or important, give it a read. And shout out to the the author on Twitter!

This is my favorite article about puzzle design, lots of useful informations from different developers of puzzle games.

Quote:
What is a good puzzle?
"I feel like when we design something clever,
players enjoy it just as much as we do." - Teddy Lee

Minimally, a good puzzle:

• has clear rules;
• has a clear objective;
• finds the balance between too easy and frustrating, to make an interesting challenge.

On top of this, an interesting puzzle can be a puzzle that:

• stumps the player for a long time, but whose solution looks obvious once found;
• uses the mechanics in unexpected ways;
• has multiple solutions;
• allows a tiered system so that beginners and novices can play at different difficulty levels.