Experience is playing the Scrum Master role for at least two years for a software development team that was diligently applying Scrum principles, practices, and theory. (Adjust based on your necessity)
Knowledge of other Agile approaches: XP, Kanban, Crystal, FDD, etc.
Awareness and experience with widely successful Agile techniques: User Stories, ATDD, TDD, Continuous Integration, Continuous Testing, Pairing, Automated Testing, Agile Games.
Applicable knowledge of the technologies used by the team.
The experience in applying a wide variety of well-documented patterns and techniques for filling in the intentional gaps left in the Scrum approach(example: various Burndown technologies, many Retrospective formats, handling bugs, etc.).