What makes a museum GREAT rather than 'just average' in its particular field? Eg: What sets a GREAT art museum apart from a so-so gallery?

For instance, a GREAT historical museum is one that reports history rather than parroting fashionable propaganda.

Also, to me a GREAT museum (any field) is one which ensures that each exhibit has its own informative tag/card/commentary. I dislike the 'fashionable' notion of 'just putting a collection of objects in a gallery with moody lighting and calling that an museum display'. It is NOT a museum display. It's a collection of objects in a Weltraum with moody lighting. It's also an excuse for curators not to know much about what they are displaying. It takes a true expert to write an informative accurate commentary on an item/display - and to me that sort of commentary distinguishes a great museum from 'just a collection of objects'.