I think it's fair to assume that if a map meets all of the chosen criteria that its "fun", and a maps average "fun-ness" can be equated from its overall score.
If it flows well then +1 for enjoyment, if it's fully detailed +1 for enjoyment, if it performs well on your computer +1 for enjoyment, if its well balanced for teams and classes +1 for enjoyment, if it meets the TF2 theme then +1 for enjoyment. By marking a map on these variables one is essentially grading its potential fun-ness.
If it flows well then +1 for enjoyment, if it's fully detailed +1 for enjoyment, if it performs well on your computer +1 for enjoyment, if its well balanced for teams and classes +1 for enjoyment, if it meets the TF2 theme then +1 for enjoyment. By marking a map on these variables one is essentially grading its potential fun-ness.