I have reviewed one of my quizzes that has been ranked "impossible". To me the word "impossible" means that it is virtually unable to be done (and only Einstein's would get a few).
When viewing the scores there were numerous people who got well above half way. The breakdown was out of 15: 15-2, 14-4, 13-3, 12-1, 11-3, 10-2, 9-1, 8-2, 7-1, 6-0, 5-1, 4-0, 3-1, 2-3, 1-4 and 0-26. I feel the 0 score significantly skews the real difficulty for there were people who got 15 out of 15.
I was going to put in more clues, etc., then decided against it. Being a brain teaser should be a little difficult (in my opinion). Lately some quizzes (in other areas) I find are getting boring for they are extremely easy due to lots of clues, can be worked out by the wording (even if you did not know the answer) for the answer is practically given to you. These quizzes are great if I want to make some quick points but that is about it. I know some editors like making quizzes easy and request extensive clues (but I do wonder how easy they should be). But I am now using the extensive clue methodology to keep an average rating.
The crux of my suggestion is that is the difficulty rating should be changed to drop off the 0's - or have a ceilling (especially when other people are getting the majority correct) to give a true rating. I think people who score 0 do not know they can back out and/or are only interested to getting a high quiz amount rather than actually trying to do the quiz. This also effects future plays, for people see an "impossible" or "very difficulty" rating and shy away.