Obvious unhelpful comments usually consist of abusive and negative comments as well as completely irrelevant ones and conversational ones. Those kind of comments are easy to spot.
But what about "incomplete" comments? Ones where someone is trying to make a valid argument but is incomplete and seems off topic.
One example is from this question. One user explains in the comments that travelling back in time is impossible due to the Second Law of Thermodynamics. The person asking the question doesn't fully understand and wonders if "information" follows this same law. Then a commenter says:
A photon is also thermodynamically constrained.
Although is may seem obvious to some that if any form of information could travel in time, it would be light; but the connection between information and photons is missing. It would sound much more relevant if the (somewhat) obvious had been stated. I tried to complete their argument in a comment of my own:
Information has to take some physical form, even if you use light, as @YuzurihaInori said, photons obey the laws of thermodynamics. So like mechanical messages, photon can't travel back in time.
But is this a case where that comment about the photons can be flagged? Can it be considered "unhelpful" if, with a few extra words, it would be fine?