The Puzzle Rating System ...
... is very simple:
Rating Sudoku puzzles strikes me as a very inexact science. I generally try to play the most difficult sounding level given - on the web that often means presumptuous names like "Super Evil, Infernal Brain Buster from Hell". In any case, from any given source I spend an hour on some puzzles with a rating like this, and five minutes on another. I will eventually implement a rating system that I hope will be more accurate than that, but for now I'm using the observation that "Easy" rated puzzles tend to be 30-something given tiles, and "Hard" rated puzzled tend to be 26 or so given tiles. Hence, a grading of levels between these extremes.