What makes a documentary good or bad?

I watched Capitalism: A Love Story. I didn’t really mean to. Sometimes you’re just curious. Anyways, it irritated me within the first two minutes.
I love documentaries. I’m really happy that The Cove won best doc at the oscars. It made me curious what makes a documentary good or bad. Personally, my favorites are ones that educate without blaming or targeting anyone. Fired!, Maxed Out, Expelled – wonderful! Any Micheal Moore movie, Religilous – dumb!