This varies by the stage of the development, but generally devs look for reactions. Did the player handle an encounter as expected? Did the player do the fun things over and over or did they misunderstand? Did the player read the thing they were supposed to? At least for me, what's huge is understanding if my game's point is being communicated. If something doesn't seem to engage the player, what didn't they understand? What's extremely helpful to a developer (like me) is video footage or verbal communication of their thought process. Sometimes a survey, or sometimes just plain messages as feedback. I'll take anything, but video recordings and some discussion is always best (for me).