I actually use Microsoft forms to gather feedback from my players when I DM, they allow for anonymous responses, essay answers, multiple choice (with fill in the blank options), or scaled answers.

If I were receiving playtesting responses, I would do a pattern of multiple choice questions followed by optional essay areas. That way if people were feeling a bit lazy you could still get some semblance of a response from the multiple choice with room followed for elaboration.

Topics that would I would consider useful for playtesting would be:

Engagement
Immersiveness
Coherence
Perceived genre
Presence of tension (or lack therof)
Organization
Grammar and spelling
Vocabulary choices

There are a lot more that I'm missing too, but those are off the top of my head.