People with whom you can be candid. "Advisors will help you will operational experience. Coaches will help you with the development of specific skills. Mentors will make you a better you" (this is from a book called 'Straight Talk for Startups').
This is a good question! I have thought about this a lot when building a service to help finding advisory and board members.
I think the dream board is about people with diverse skills and values that are in align with the company values. It's also important that they understand something of the company's current status, I mean that startups usually have a bit different style and needs than big corporations for example.