1. Fiction has to make more sense.You've heard the old adage "truth is stranger than fiction." What that means is that while in real life coincidences and strange, random events occur all the time, in fiction they seem out of place. While real life is often chaotic and random, and you run in to your old kindergarten teacher on the street or someone gets hit by a bus without cause or reason, fiction has a greater sense of order. Fiction is often used to make sense of the things that happen to us. Non-fiction, too, must give us an argument or a story to follow, but it can have more leeway in this regard. An essay about the life of an individual, for example, can end with his senseless death.
2. Fiction needs more direction and purpose.As noted in the previous point, fiction is often seeking to make sense of the random or chaotic way life seems to operate. It may conclude that we can't make sense of the thing, but it will still make that conclusion in an orderly way. Non-fiction, on the other hand, can report on an event or a life in a more neutral way. It doesn't have to recast events in a way that will better serve its thesis. It may sometimes, but it doesn't have to.
3. Non-fiction amazes with the facts: fiction amazes with the story.Another way non-fiction relies on different machinery is in how it actually entertains us. Both forms can have great writing and apt observations. However, non-fiction has our trust, and it can amaze us with the facts. If it tells us that something incredible happened, we are stunned because we know it really happened. If a piece of fiction just blurts out something incredible, we are unmoved; it's made up. Fiction amazes us with story, by delighting us with how that incredible thing came about.
So if you have a great story to tell, consider carefully what form would suit it best. Do you want to alter and polish it, shape it to serve a story, or let the facts rest on their own? Do you want to include your own reflections, or disguise them in the voice of a character? Make sure your story is best-served by its form.