What makes a documentary good or bad?

The worst thing someone making a documentary can do is appear in every frame throughout the entire film. A documentary should tell a story about something else, not be a tribute to yourself. I watched a documentary once where the whole film featured the same guy, the documentary maker. Sucky sucky.

if it adds knowlege and value to the watcher

A good documentary leaves out opinion and speculation and sticks to the facts.

Subject matter & presentation.

I would argue that a good documentary can either challenge or reinforce your beliefs about something. I don’t think there is one formula that determines worth. Film-making is by nature subjective; the “facts” you include or exclude support the position you want to convey. A good documentary, like any work of art, presents stimulating material in a visual way that provokes a response.

My two criteria are a subject matter that interests me and factual presentation, where any opinion or speculation is clearly identified as such. The “attack” documentaries leave me cold, as athe producer clearly has an agenda and his picking his/her “facts” selectively.

It makes you interested. It makes you care. It is well produced. It is well researched. It moves you.

How relevant it is to you.