The act of playing the game is totally fun to me.  The mechanics feel good and the designers change them up a bit as you play.  Not only does each world have its own signature time mechanic, the puzzles in which they are used seemingly become more impossible as the game wears on.  But, these puzzles are always simple and organic to solve.  For instance, through observation and simple tinkering, one can solve any puzzle in the game.  The game never requires ludacris luck or guessing to solve the puzzles albeit probably seems to.