Cite examples for the points you make.

This does not mean rewriting their script.

For example, a reviewer might write, “you used very expositional dialogue”. The reviewer should give the best (or worse) example of the "expositional dialogue" they found in the script.

No writer sat down and thought, “I’m going to write this bit of dialogue with a lot of exposition”. 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 have a choice about it.

Be sure not to use jargon that goes no further than being a simple label. For example, a reviewer might write, “the hero struck me as a stereotype”. That begs the question, what kind of stereotype? Was any attempt made to not have the hero look like a stereotype? If so, why did it fail?

Another example, “everything came too easy to the hero. You’ve got to make things difficult for him!” This begs the question, what was easy?

If you’re an advanced reader/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 set up the hero as a good guy you would like. 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 set up the hero as likable. Why did it fail? What subsequent actions on the hero’s part undermined it?

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 it didn’t work!

“I couldn’t empathize with the hero because he used steroids in the past.” That sounds like a smart comment with proper use of jargon, but it’s actually totally flimsy.

A flawed hero is a staple of storytelling! Use the tried and true “compare and contrast” to make your point. For example, a good reviewer might write, “Rocky was a bum, but he tried real hard to be good. Your hero is just a bum and seems ok with being a bum. I have a hard time empathizing with that.”

Without examples, you’re just beating them over the head with complaints. They can’t remedy their mistakes because they don’t know where they occur.

If you don’t know why you don’t like something or don’t have a good reason, it’s totally fine. Just acknowledge that it’s a gut reaction in your review.

And finally, it should go without saying, never try and show off how clever you are by picking apart someone’s writing with wit or sarcasm. 99 times out of 100, it isn’t clever and even when it is, you just sound like an a-hole.