We would agree with all the above comments and we too are fine with the current star system - one area that could be improved would be for airbnb to ask why a lower rating was given.
We get five star reviews 92% of the time (what's causing 8% to give us four?) a great example of 'mood' being a factor is our most recent four star rating: the guest gave us five stars in all six categories.
5 Stars for: Cleanliness, Accuracy, Value, Communication, Arrival, and even Location. He gave us 4 stars for over all experience!!
It would be appropriate for airbnb to ask prior to feedback completions:
"We see you have five stars for every category (or even five of six categories) can you tell us why it was a four star experience?" This might help people re-evaluate why they are 'dinging' a host who's usually worked hard to make sure their stay is comfortable.