This point might sound obvious and unimportant, like saying "make your game fun", but it's actually a crucial part of my design process. Forgive me if I labour the point.
Be constantly aware of your emotions during playtests.
Normally, we're trying to ignore negative emotions — not monitor them. In day-to-day life, we don't haul a bag of groceries, and analyse ourselves as we do it. We don't think "my hand is sore... my arm is sore... I'm somewhat frustrated..."
It's a strange thing to do, but if you're experiencing a negative emotion while you're playing your game, other players will too.
You're creating an emotional experience here, and you can test it on a human — yourself.
Earlier in the development of Radlands, I noticed that I had more fun when there were less cards on the table. When there were more cards, the game could become a slog, and it was hard to impact the opponent. While the slog was entirely fair, I found it frustrating and boring.
Emotional response is a very blunt and unconstructive kind of feedback, and requires an advanced level of self-criticism. It's hard to say to yourself "I was bored", when you've just played your own game, that you've put so much work into. It's much easier to make excuses as to why it was boring, or pretend it wasn't, or justify that the game is good anyway.
It's easier when you're a good designer, and are comfortable with this process.
Play your game, and take note of your emotions.
Also ask your playtesters what their feelings were, after your playtest. It can be hard for them not to give technical details, or analysis.
It takes time to drill this attitude into your playtesters, and then have them be honest about what they actually felt. Most playtesters simply cannot separate analysis from feelings. Most playtesters need encouragement to say "I felt powerless" or "I was frustrated."
When asked what they liked or didn't like, they tell you what would or wouldn't be fun, or tell you about balance. They also take the noble position that maybe they just don't get it, and you know this design stuff better than them. They also feel like they should also offer solutions, which are typically not useful. I have to specifically tell them that I really just want them to complain about things.
Feelings are never wrong.