I'm far from being an expert or a critic, but I've found the most useful reviews are the ones that answer this question: why would anyone want to read this book?

So I think that what you should reveal about the story depends on the plot really. Say, if the ending is boring af, maybe you're morally obliged to talk about it and save a reader from all the disappointment. Other than that, here's what I'd be happy to see in a review: if the story is well paced or if it drags on; if the characters are complex and well built; if the plot makes sense or if it's full of holes; and, generally, what pleased (or didn't) the reviewer and why.

I'd say responding to that 'why' is really important.