Comment Re:Value (Score 2) 44
Absolutely spot on.
I wrote a Calcudoku generator/solver a while ago (now in heavy use at http://www.calcudoku.org/ and spent a _lot_ of effort on the difficulty rating bits (and still know there is room for improvement).
One idea that isn't in there yet is to somehow incorporate the distance between the solving steps: if the next logical step is in a cell very near the previous one, you see it more quickly, hence the puzzle is easier.