Players hate change. Be aware of the baby duck syndrome when designing your game. Follow genre conventions.
Make player damage juicy: Use UI iconography, text size, colour coding, timing, and audio to show feedback of impactful player damage in your combat designs.
Use observation systematically to study your users in the field.
Label unfamiliar icons. Make your game UI accessible. Destiny 2 is a great example of labelling UI elements that are not self-explanatory to the player.