I have to disagree with you here. This is likely the last time you'll see your guests, at least in this lifetime (do you get many return guests?), so that's the last chance you get to explain stuff to them. Do you just have them leave the keys on a table or something?
There are a couple of alternatives.
1) Meet guests for a sort of 5 min "hail and farewell" session. If my guests are leaving early, I meet them the evening before. I take their keys then, and ask them to fill in a feedback/suggestions form. Most of the time they do it. That's when I give them a little lecture about the meaning of the star ratings.
2) Additionally, or alternatively, you could work some comments about the star ratings into your guest guide. There's a thread on that somewhere - I think @Sarah_Warren did it. Does anyone have a link handy?
I was sort of kidding about the photos. But it's certainly a good idea to have the actual apartment looking as close to the photos as possible.