i think what makes a great story is different in fiction and nonfiction. made up stories have to actually seem true. the thing i love about nonfiction is: it does not have to be plausible. i mean, great nonfiction is all about coincidences and things that don't seem like they really happen. when i was writing the assassination book, one of my favorite stories was about edwin booth, who was john wilkes booth's older brother. he was at a party once, in new york. the host of the party had this plaster cast of a hand on the mantelpiece. and edwin booth picks it up and asks the host "whose hand is this?" and the host says, "uh... lincoln...you know, like, that guy your brother murdered..." stuff like that. if you put that in a novel it would just be so hamhanded and ridiculous, but because it happened, it was allowed.
and yes, i did have an entry earlier this year on the booth family and synchronic coincidences.