What is most important? Other than a budget?
Time: lots a time, to think, read, to 'let it rest', to 'talk it out', let it simmer and tweak it. Or maybe: access. I always hear that access is the single most important thing in doc film making - an unique story and access to that story - personally I would also add relevance to that list. In my experience I would say "access" can be replaced with "stubborn perseverance". It worked in this case. Oh, and a whole truck load of luck.
A really thick skin (best a fur) is essential too -especially after you've finished the darn thing and put it out into the world. There is fine line between good advice and some one's opinion. One you accept and make changes the other you listen to (agreed or not) but leave it at that.