Experience as a business leader.
Your executive coach should have been in a formal position like a CEO or a Managing Director or a President of a business organization and he or she should have served on the board of directors of this organization.
This is important. A view from the top is fundamentally different. No amount of lateral experience at lower levels can give the perspective which a top leader gets through his or her work. Only a person who has had a view from the top can help you no matter at what position are at now.
If you are a CEO, then the above requirement is even more critical.
Additionally, your executive coach should have had functional experience that straddles at least two of these areas: marketing sales, operations, finance, product creation, R & D.