There is something I have always “known” but only recently began to truly appreciate. That is: when making a game get feedback from an outside source whenever practical. The reason for this is simple: as the creator, you aren’t going to see things as clearly as someone outside of the project. And I had always “known” that, but only in my last project have I really lived it.
I have frequently been trying to get playtest feedback from people. But it was always in limited amounts. And the feedback I got didn’t have much in the way of substantive changes to make to the overall experience. But just recently I got my game in front of a slightly larger group than normal and got some feedback that made lots of sense. Things that I hadn’t been thinking about, but when I gave them 3 seconds of thought made perfect sense.
The feedback you get from people outside your project helps give you a new perspective. In my case I am applying the feedback I got pretty directly. But in many cases feedback can serve to force you to ask a critical question: Why did they have that response? By reflecting upon that question you can find issues that you have been blinded to by your familiarity with the project. These problems might not even have any direct relation to the thing the feedback was about. But changes elsewhere could have cascading effects that eventually change what was complained about.
Overall, feedback isn’t about “You need to change thing X because Y” but about seeing things from a new perspective. That way you can pull back and take in the whole thing, rather than getting lost in the weeds.