Main plot points? Setting? Cast of characters? Tale told chronologically or not? Sub-plots? Themes? Story arcs? Emotional journeys? The list of choices is endless.
One of the first decisions to be made is on whose story it is. My august mentor, Anita Burgh (who has her own blog at anitaburgh.blogspot.com), is insistent on making sure all her protegés know and understand this. Usually, we establish the main character right up front (and normally end with him or her), so making sure the reader is completely engaged in the plot line that will drive the book. Clearly the 'owner' will have his or her own point of view (POV). If you are writing in the first person, this is the only point of view you will be able to use throughout the whole book. Personally, I find this quite challenging and prefer to use several viewpoints, so that I can come at the main plot from a number of different angles.
If you are using multiple viewpoints, however, you have to be very careful. Anita taught me to be absolutely rigid in this, always making sure there is a line space between switches of viewpoint at the very least, so that the reader can follow whose 'head' you are in. Also, it's important that every character who has a point of view also has a story arc, so that the reader is engaged his this line of the story.
Should there be rules in fiction writing? You could argue that rules rather undermine creativity and freedom of expression, and perhaps they do. But as a reader, I would strongly support the view that 'head-hopping', as it's known, is a no-no. I recently read Family Album, by Penelope Lively. I really enjoyed her fine evocation of character, but at one stage I was left utterly bewildered during a round-the-table scene where we were in the head of each and every one of her characters in the space of a paragraph.
Can you break rules? Of course. Rules are always there to be broken. Elizabeth Chadwick recently tweeted that she had decided to change the POV briefly within a sentence because it worked best that way. Sometimes it's a judgement call – a complete POV switch could seem clumsy or overly intrusive in a particular context – but for my money, less experienced writers should stick with the rules.
That's my point of view, anyway!