What would be a good very short conclusion to a review of Shakespeare's Hamlet?
Without knowing exactly what your play review is focused on, this is difficult. Did your review praise the play? Did your review condemn the play? Or a mix of both? Did your review focus on a specific character or series of events? A conclusion is meant to summarize the body of your writing and give your reader a final push into caring why what you wrote, or why what you wrote is important.
Perhaps the following is what you are going for?
"Although Hamlet is one of Shakespeare's great tragedies, the lack of any redeeming characters really sours the play for audiences. It is a great story of revenge and intrigue, but most of the characters are motivated by selfish ambition, which makes the overall tone of the play quite dark. Shakespeare wrote other tragedies, and those are the plays that readers will like more."
It's more than 20 words, but 20 words is tough to get down to. It's also very vague and general. Like I said, I have no idea if you are praising the play or not.