Employee appraisal is broken and while we all can blame the bell curve, the truth is that most of the companies tend to create too many parameters for evaluation leading to confusion across the board.
What if there was just 1 parameter?
How about Re-Hire as a metric to evaluate people (and vice versa)? That is :
Would you like to re-hire this person ? For same role / same salary ? Would you like to work with this person again?