You have an ethical responsibility to inform playtesters if your game could foreseeably make them uncomfortable.
— John Brieger
Don’t sacrifice initial playability in service to replayability. If you add so much variance to create different scenarios that the first game is sometimes a non-game (unwinnable or unloseable), those players won’t bother with a second play.
— Jay Treat
Find your own niche and then own it. Each thing you make is pushing yourself in a new direction. Don’t just push along one axis constantly and use one perspective of what ‘good’ means.
— Behrooz "Bez" Shahriari
You don’t need to tell playtesters if you’ve already tested a suggested solution. Refocus and keep them talking.
— John Brieger
Have patience. Lots of patience. Developing a game until it is good can take many years. Is a publisher interested in your game, then you still need to have patience. Keep in mind it can take months, sometimes years before a game is published.
— Dennis Merkx
To ensure a steady stream of playtesters, you need to engage with your community. You can’t have all your interactions be you asking them to testing your games.
— John Brieger
Customers will likely play your game more in the first month of its release than all of your playtesting combined.
— John Brieger
There are many reasons to make games — making a commercially successful product is only one measure of success.
— John Brieger
When writing game rules, be wary of VAST COSMIC FORCES that occupy the negative space when you don’t identify the actor in a sentence. Example:

”The deck is shuffled.”
BY WHOM?...
BY VAST COSMIC FORCES!

Try “Shuffle your deck.” or “Each player shuffles their deck.”
— Daniel Solis