That said, it's worth trying to sort out what makes a strong opening line. In a sense, it's a pick-up line, and you want it to be suave and genuinely intriguing, not cheesy or a turn-off.
1) One of my favourite opening lines, written by Stuart Neville:
"His hands just looked dirty to casual eyes, a slight darkening on the knuckles, a shadow on his palm."2) Here's (my best stab at remembering) a tweet from Livia Blackburne that I thought would make a good first line for a book:
"That is the LAST time I go to a conference without my wedding ring."3) From EIGHT BLACK HORSES by Ed McBain:
"The lady was extraordinarily naked."To me, the one thing that ties all these together is that the line implies a greater story, and an apparently intriguing one.
Mr. Neville's line implies a tension between apparent civility and uncivil violence. His character has obviously been fist-fighting, but the language implies you could be sitting right next to this fellow and not realize he's capable of brutality. It's a creepy thought, and it implies this character has secrets and a dramatic life, and thus might be interesting to know more about.
Ms. Blackburne's line makes you think you might know what sort of occurrences led to her statement, and you sense they might sound pretty funny to someone who didn't have to live through them. That promise of amusement works as a lure to make you want to hear more. The reader senses the emotion the speaker is feeling (exasperation) and assumes dramatic events must have given rise to it. We become hungry to hear about the drama.
Mr. McBain's line implies this character who is seeing the naked woman is pretty shocked by it. After all, she's not just naked; she's a "lady" and "extraordinarily naked". Given nudity is a binary state, and you can't get more naked than naked, the reader ends up curious as to why this observer is reacting so strongly. Again, we sense his emotion and we become curious to know what drama is provoking it.
Moments of struggle, moments of conflict, and moments that provoke high emotion constitute "stories". Nobody wants to hear about how the photocopier worked normally today--they want to hear about your epic battle of wills with a hostile and diabolically cunning photocopier.
First lines have to firmly imply a story. This means they need to imply a conflict on some level. It can be a subtle conflict created mostly in the reader's mind, such as the one formed when Mr. Neville hints at the difference between what you see and what is actually true about this character. It can be an external conflict such as the one we suspect lies behind Ms. Blackburne's statement--we assume she was in an epic battle of wills with someone (or several someones) at the conference. It can also be a internal conflict such as the one between what the character was expecting and what they actually got, as seen in Mr. McBain's line.
A second thing to note, however, is that a reader isn't going to necessarily care just because a conflict clearly exists. You need something more to rope them in, to make them decide to invest their time in the story.
What does the job? Any number of things, and here are a few: Empathy, curiosity, humour.
Mr. Neville's line provokes your curiosity. The author sets the scene in such a way you might be sitting next to this character on a bus. You glance over at his hands and abruptly realize something doesn't add up. You had assumed this person was harmless and boring (like most of us are), but he's been brawling. Suddenly, you're curious. Who exactly am I sitting beside? What mischief has he been up to?
Ms. Blackburne's line implies humour. She might not think what's happened to her was funny, but you know you probably will and she sounds ready to vent and thereby let you have your amusement. Thus, you decide to stick around and hear more of her story. The humour you anticipate is what lures you in.
Mr. McBain's line provokes empathy. You don't know a thing about this situation beyond the fact the observer is seeing a naked lady and feeling very shocked. All that's drawing you in is an emotion, a reaction you can relate to. You are curious about the situation too, but what provoked your curiosity was the fact someone else (a fictional someone) cared about it. In effect, the character's emotion acted as a testimonial saying, "This is worth caring about."
In summary, a first line needs to promise some kind of conflict. You don't need a murder victim or an explosion in that first line (although you can have either), just some tension between two things in opposition. If your sentence implies something interesting or rewarding is about to be presented, the reader will stick around to see what that is.