A helpful feedback is thoughtful and honest. It's specific, and provide a clear path for the speaker to improve his or her next presentation. It should have examples and suggestions.
Why honesty is important
It's because honesty is rare. One star does not mean it's harsh. It means that compared to other aspects of the presentation, the area with one star is the one that needs the most improvements.
The five-star rating system was picked for its simplicity, but it suffers from a lot of biases, for example, reference bias. To illustrate this point, imagine two speakers: speaker A is a professional public speaker who has travelled the world, and speaker B is a new speaker just starting out. Speaker A asks feedback from critics, therefore getting 2-3 stars mostly, whereas speaker B gets feedback from other people who's just started in public speaking. Speaker B will get 4-5 stars.
It's consequently important to not use the rating system to compare between speakers. It's useful, however, for the speaker to compare his or her performance from one presentation to another. The speaker would most likely have a similar set of feedback givers, therefore it will be fine to compare the rating between presentations.
What is unhelpful feedback?
A feedback that is not specific, or referring to things that the speaker cannot change such as race, or gender is not helpful. It's also not helpful to rate all areas with the same stars because then the speaker does not know where to improve.