only how many tries it take a solve level. Example: many sokoban style levels
(like Temp 15) are easy to solve, but takes so many moves that medium is a
correct rating. A level rating should be compared another similar levels, not
necessarily all levels. I mean that a "pure" sokoban level is always easier
than a really deadly level that uses all objects, but that doesn't mean that
sokoban level could not be rated deadly.
level is solve. The most important thing is that ratings are consistent with
each other. Since Jim has rated most levels, you should probably use the same
rating principle that he used, whatever that is.