For me the best opening lines should have three key elements: brevity, paradox and thematic projection. And if that line is also subtle — that is, you may initially miss the implications — then it shines.
One such shining example of great first lines is that from Ray Bradbury’s “Fahrenheit 451”: “It was a pleasure to burn.” At first glance, a reader may miss its brilliance. But as one gets into the story, it all becomes clear what Bradbury accomplishes in that hook of an opener. The novel is about a future dystopian society where books are banned and where firemen torch libraries — a society where people are dumbed-down to raw instincts.
What makes the opener brilliant is the double meaning of the verb “to burn.” It is both a transitive verb (has a subject and direct object) and an intransitive verb (does not have a subject and direct object). In short, “to burn” means to set something ablaze as well as to be on fire. So the opening line projects the brutal paradox and philosophical core of the novel: to burn books is to self-destruct. And instantly dramatizing that is the long rich paragraph that follows, descriptions of protagonist-fireman Guy Montag’s almost-sexual pleasure of flame-throwing books as their pages flap like bird wings while being scorched. Next, without thought, Montag takes a near suicidal plunge down the firehouse pole, stopping himself just inches before crashing to the floor.
More importantly, the opening line is delivered from the point of view of Montag who will experience an arc of humanization throughout, moving from a mindless burner of books to a rebel who fights the dehumanizingly censorious system.