I often think this is a disservice because there are many things about a production that can be worth talking about even though they are not the main thrust of the show and even when they don't effect the overall quality of a show.
Sometimes I think some of the more interesting moments in a show are the small ones that go by quickly. They can be great moments, or they can be horrible ones. When they're bad, mentioning them seems like an exercise in picking nits. They can be interesting to discuss, but really shouldn't effect whether someone goes to see a show or not. Sometimes the little things are matters of interpretation--one person wondering whether a slightly different choice might have been more effective. In the latter instance, the overall performance may have been quite good and it can be demoralizing to raise a question in a review.
At other times, the nits do matter. Sometimes the lack of attention to a small detail can take the audience out of a production and make the overall show suffer. Sometimes the jarring detail or moment can indicate a sloppiness on the part of the actor, crew, or director (depending on what it is).
I've been bothered lately by one of those small details I saw at a show--a detail that would likely go unremarked upon in a review because there were so many more important things to comment on. And yet, that detail told me a lot about the show and about the actor. In the play, the actor is complaining about how cold it is and how frozen he feels with a lack of proper heating during a winter month. He's wearing a scarf and a sweater and would rub his hands to keep warm. Yet he had his sleeves pushed up to his elbow. It immediately told me that as an actor, he wasn't paying much attention to what he was saying. He wasn't feeling the cold. He wasn't thinking. I couldn't believe what he was saying because if he were really cold and shivering, he'd pull his sleeves down rather than leave his bare arms exposed to the frozen air.
Does that minor point of costuming matter compared to the other elements of the show? Only in that it informed my thinking about the other problems that show had and what might have contributed to them.