It is not necessarily the plot that's important. I wouldn't even say it's those parts that you view as "boring" (boring could mean it's still important, but it needs to be rewritten).
James N. Frey's How to Write a Damn Good Novel (Chapter 3), recommends that you use a story's premise to be selective in determining what goes in and out of your story. The premise of a story is essentially "a statement of what happens to the characters as a result of the core conflict of the story." For example, the premise of The Godfather can be expressed as "family loyalty leads to a life of crime", while The Old Man and the Sea has the premise of "courage leads to redemption".
You likely will not know the premise from the start, but it should become apparent from your characters, how they will likely interact with one another and the conflicts that arise, which ultimately reaches a conclusion. As Frey puts it, "There is no formula for finding a premise. You simply start with a character or a situation, give the protaginist a dilemma, and then meditate on how it might go."
However, once you do know your premise, you can be ruthless in removing all that has no bearing on proving the premise, which ties in nicely with the first item in your list. Like a sculptor's knife that strips away excess clay to reveal the statue, the premise whittles away the cruft from your story. As Aristotle puts it, "For that which makes no perceptible difference by its presence or absence is no real part of the whole."