We present results from a study that categorizes and assesses the quality of questions and explanations authored by students in question repositories produced as part of the summative assessment in introductory physics courses over two academic sessions. Mapping question quality onto the levels in the cognitive domain of Bloom's taxonomy, we find that students produce questions of high quality. More than three-quarters of questions fall into categories beyond simple recall, in contrast to similar studies of student-authored content in different subject domains. Similarly, the quality of student-authored explanations for questions was also high, with approximately 60% of all explanations classified as being of high or outstanding quality. Overall, 75% of questions met combined quality criteria, which we hypothesize is due in part to the in-class scaffolding activities that we provided for students ahead of requiring them to author questions. This work presents the first systematic investigation into the quality of student produced assessment material in an introductory physics context, and thus complements and extends related studies in other disciplines.