First things first: CoG’s post on beta testing has lots of helpful guidelines.
Some authors do, some don’t! One author in particular - just the one - responded to my feedback point-by-point. That was so much fun that I’ll happily test for that author till the sun rises in the west. Not everyone has the ability or desire to do that though, of course!
Other authors have occasionally sent me a little thank-you note. Some authors I haven’t heard from at all. In that last category, sometimes I’ve noticed the change log includes every single suggestion I’ve made (that was kind of amusing). Other times I haven’t noticed any of my suggestions being implemented, outside maybe the typos, across several drafts.
I do have to be careful because I’m an editor, and editing-brain is not the same thing as beta-brain. I try to focus on typos, anything I found immersion-breaking, and suggestions that make it easier to role-play the character I envision.
As for my perspective as a (read: developing, hopeful) author - I appreciate all of the positive feedback, because it encourages me to keep going on a very long game. But as to what was most helpful? So far, several wonderful people have spotted typos and continuity errors, and that was amazing and brilliant! That’d be the low-level feedback mentioned in the CoG post above.
Then there are two specific pieces of high-level feedback - plot, content, characters - that were extremely helpful to me. One was a suggestion to add an extra level of mechanics to one scene (letting people pick extra ice cream flavours, of all things, and it’s turned out to be a very popular addition!). The other was when someone outright told me one scene was very boring. That person, after some thought, told me why the scene felt boring - not enough plot, too much fussy automaton, and not enough action by the MC. I’m slowly working through it now, addressing those issues, and I feel like the scene is much stronger for the additions.
That said, I’m really relatively recent to Choice of Games compared to many on the forums. And I’ve yet to publish a piece of interactive fiction. I’d love to hear the perspectives of some other beta testers and authors!