On the outset, I would say that any answer here is taken in its most theoretical form. The reality might differ based on organization needs, institutional pressure, and perhaps, even personal politics in the workplace. This is not to sanction such realities, but rather suggest that human resource networks struggle with the gap between theory and reality in many work settings.
I would say that one of the first elements of a successful performance appraisal would be to do what is necessary to embrace the idea of 360 degree feedback. This makes appraisal something reciprocal, whereby the assessor and the individual being assessed conduct an articulation that is more dialogue than monologue.
It is important for any employee to leave an appraisal feeling that there is room for improvement, areas upon which to focus, and a feeling that there is a solid foundation for growth. I think that 360 feedback guarantees such elements.
Any assessment of performance appraisal should be supplemented with data, evidence, and not be isolated or targeting the individual. Performance appraisals can devolve quickly into accusatory forums if there is a lack of data and substantiation. It is important for such articulations to not be "gotcha" settings where an individual feels personally attacked, but rather supported into being the best professional possible.
Finally, I don't think that there is anything wrong with trying as hard as possible to make performance appraisals motivating. Instead of seeing it as an opportunity to browbeat an employee, what if an appraisal of one's performance can actually be motivational to improve? If the employee does leave feeling inspired and motivated, the organization, the appraiser, and the employee all benefit. I tend to think that this is something missing from the appraisal process and could serve as a good "rule of thumb" to keep in mind for conducting a successful appraisal of an indivdual's work.