PEN America's David Haglund has some thoughts on the rules surrounding "spoilers" in critiques of movies and TV. Here is the last line of his piece on The Awl:
And if a work of art can be "spoiled" when we know the ending, it can't really have that much to say about life, can it?
I agree, and he makes a good point about Citizen Kane, though I want to give Orson Welles the benefit of the doubt, preferring to see the ending as something of a joke on moviegoers expecting a tidy resolution.
I avoid spoilers for TV series that are buzzworthy among critics or among my acquaintances because it's fun to speculate, between episodes, on where the series is going next. For movies (and plays), I almost feel at a disadvantage not knowing the ending because I can't tell how effective the filmmaker is at reaching that ending. I can forgive a boring film more easily than one with a twist ending that, upon reflection minutes after leaving the movie theater, negates almost everything that came before it.