I had some guests gave me a 4 star in cleanliness because they found two hairs on the bathtub. But the whole apartment is super clean. I personally would not take one star off because of two hairs as I think one star means 20% off which is too heavy for two hairs found . But the guest thinks it differently. He thinks as long as it's not perfect he would give a 4 star in cleanliness. But you can always find imperfections in any businesses. This rating is very unfair.
I hope that Airbnb should pop up a guideline message when guests are about to give ratings. For example, 5 star cleanliness means 81%-100% of the place is clean. 4 star means 60%-80% clean, etc.
For overall ratings, they can give a guideline likes 5 star for perfect shower facility, polite and friendly host, noise level is good, etc. And 4 star means blah blah blah
That would give more fair ratings to every host and it would not cause the picky guests got cancelled and unwelcome when they book the same place the following time. Everytime I realize the guests are too strict in giving ratings, I would message them and tell them I would not accept their reservations in the future. Getting one 4 star really hurts as once I get a 4 star I need to get four 5 stars in order to reach the super host standard. And not every guest gives reviews which means once I get a 4 star, I need to get at least 6-7 guests in order to hit the super host standard. Therefore, it is better to not accepting guests at all rather than accepting a 4 star guest.
Using guideline doesn't mean all guests would follow this but I believe this can help in certain extent. Hopefully every guest would rate using the same standard.