The number one attribute of good feedback is citing examples for the points you make. This does not mean rewriting their script.

For example, a reviewer might write, "You used very expository dialogue." The reviewer should then provide the best (or worst) example of the expository dialogue they found in the script. No writer sits down and thinks, "I'm going to do a data dump of what led up to this scene using dialogue." In the writer's head, it was natural and necessary for the characters to say what they said; otherwise, it wouldn't have been written. Examples give the writer access to the reasons behind your thinking. They can then agree or disagree with what you said and make an informed choice.

Be sure not to use jargon that serves as nothing more than a simple label. For example, a reviewer might write, "The hero struck me as a stereotype." That raises the question: what kind of stereotype? Was any attempt made to avoid making the hero look like a stereotype? If so, why did it fail?

Another example: "Everything came too easily to the hero. You've got to make things difficult for him!" This raises the question: what exactly was too easy?

If you're an advanced reader or writer, you should be able to see where the writer was attempting certain things in the script. For example, there's usually a scene that exists primarily to establish the hero as a likable person. If you, the reviewer, don't like the hero, it isn't enough to say, "I didn't like the hero or care if he succeeded or not." What needs to be addressed is why you don't like the hero. Cite examples and specifically address the scene intended to make the hero likable. Why did that scene fail? What subsequent actions on the hero's part undermined it?

Consider another example: "It's a murder mystery, but I didn't feel any suspense." Was there a setup that tried to create suspense? There must have been! Address why that setup didn't work.

Writing something like, "I couldn't empathize with the hero because he used steroids in the past," might sound like a smart comment, but it's actually completely flimsy. A flawed hero is a staple of storytelling! Instead, use the tried-and-true "compare and contrast" method to make your point. For example, a good reviewer might write, "Rocky was a bum, but he tried really hard to be good. Your hero is just a bum and seems okay with it. I have a hard time empathizing with that."

Without examples, you're just beating the writer over the head with complaints. They can't remedy their mistakes without knowing where they occur. Without specific examples, you're just pointing at the entire screenplay and saying, "Some dialogue is unnatural," "Some scenes start too early and finish too late," or "Some jokes fell flat." The writer is then left with guessing which parts need repair.

It's entirely appropriate to expect a reader to jot down notes while reading. If a reader can't be bothered to do that, they are unreliable and should not be trusted or sought out for future reads. If the reader doesn't know why they didn't like something, or doesn't have a solid reason, that is completely fine. It should simply be acknowledged in the feedback that it is simply a gut reaction.

Finally, it should go without saying: never try to show off how clever you are by picking apart someone's writing with wit or sarcasm. Ninety-nine times out of a hundred, it isn't clever and even when it is, you just sound like a jerk.