There are two steps to assessment: you gather information about student learning and you share that information with others such as the student, parents and administration. Occasionally, you might add the hint of an evaluation, but otherwise students should experience their successes and failures not as reward and punishment but as information. Because homo sapiens can only ever experience grades as reward or punishment, we don't need them to gather or share this information about student learning.
The worst kinds of assessments encourage students to do only what they are already good at and hide their mistakes so that the teacher can judge. The best kinds of assessments encourage students to take risks and make their mistakes transparent for the teacher to use as a way to plot future learning. In other words, assessment is less about gotcha and more about diagnosing learning.
Just like learning, assessment should be about working with kids to construct and apply their understandings to create and perform for a purpose and in a context.