I think that a typical documentary has a lot of interviews with professionals and maybe members of the public. I also think that a presenter or a voice over can also be good in a documentary. Visual facts or reconstructions are always good as different people have different ways of learning so a mixture of visuals can be important.
Personally I like nature documentaries such as, Africa or Frozen Planet as documentaries like that give us a chance to witness something that we will most likely never see. Also I think a well respected presenter like David Attenborough makes me more likely to watch.