Test the ‘final’ game with people that have never seen you play it or had you explain it.
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It’s about identifying accidental design slips that you would likely apologize over if you saw someone struggling with it.
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This is about having one last chance to smooth over, fix, or improve the places that new players get stuck or confused by when sat in front of the game without help. If someone needs you as the developer to explain, clarify, coach, or point something out to them when they sit down to play the game, multiply that problem times 1,000 players and realize that you won’t get an opportunity to talk to any of them.

Chris DeLeon (via notgames)