A well-done rubric is both an instructional tool and an assessment mechanism. Here is a list of characteristics to strive for to create a purposeful rubric.
An effective rubric must possess a specific list of criteria, so students know exactly what the teacher is expecting.
There should be gradations of quality based on the degree to which a standard has been met (basically a scale). The gradations should include specific descriptions of what constitutes "excellent", "good", "fair", and "needs improvement". Each gradation should provide descriptors for the performance level. Typically there are 4-6 gradation levels on a rubric.
Effective rubrics offer a lot of descriptive language. The rubric describes exactly what makes an assignment quality. By specificity, the descriptors enable student performers to verify and comprehend their scores.
The difference in quality from a score point of 5 to 4 should be the same difference in quality from a score point of 3 to 2. All descriptors should model and reflect the consistent levels of continuity.
A "good" rubric should be able to be used by various teachers and have them all arrive at similar scores (for a given assignment). Reliability also can refer to time (for example, if you are scoring your 100th essay - the rubric allows you to judge the 100th essay with the same criteria that you judged the 1st essay).
A rubric possessing validity, scores what is central to the performance and assignment, not what is easy for the eye to see and simple for the teacher to grade.
Don't forget to model exemplars of products at various achievement levels (be sure to keep the models anonymous).