To satisfy our curiosity about that particular puzzle, the common words found by the fewest players were:
NAMEDROP 10
ENAMORED 33
DEMEANOR 39
POMANDER 45
MONAD 49
DAEMON 54
RANDOM 75
Interestingly, MONAD is the only one of these queried by my spell-checker. ENAMORED and DEMEANOR both have an -OUR spelling in places where British style spelling is followed, so that may have caused some players to miss them.
Quite a few "rare" words were played by many more people than any of these, such as:
DOPER 185
MOPER 169
OARED 130
PONE 121
REPO 107
DORE 104
AERO 100
Some of these, I think are "Chi-words", that is, words that people have got to know from the puzzle, rather than from usage.
MK, I like your idea of making this sort of information available when looking at any solution, but I'll have to look into the practicalities of it.