The beginning of a novel--ideally, the first few lines, unless we are in a prologue, about which more next time--should open a window and set a scene immediately. Readers should be able to see, hear, smell, maybe touch, and sometimes taste the setting and surroundings very soon, if not right away (because we're in a character's mind, or we're observing something closely, frex.)
Readers must know within moments who the main character(s) is/are and the situation at hand (the story world, the "ordinary world," if you will). Also, the TONE of this opening paragraph needs to signal to the reader whether this will be a light-hearted romp or a heavy, philosophical, convoluted read. However, this is all preliminary to "THE STORY." A novel doesn't have liftoff (even if we have readers already immersed in the Vivid, Continuous Dream that their minds should be constructing) until we pass through the Doorway of No Return.
What do I mean? Well, we're about to lose that ordinary world. The footing is unsure. A chasm is about to open beneath our feet. There is a door standing ajar, and perhaps a monster in pursuit of Our Heroine. Once she accepts the challenge and steps through the portal, she won't be able to get back home until the major plot problem is solved AND she has changed, fundamentally, herself. (A book has to be about the most important thing that has ever happened to the main character, and she has to undergo a transformation. Ideally, she will face her worst fear and conquer it. This doesn't hold for mysteries and series blorfs, but it SHOULD.) This challenge should be before the 20% mark or first 1/5 of your book, or else readers will get bored and drop the book. It really should be much earlier than that, for today's readers.
Is your hero or heroine intriguing, charming, worthy of following for 200+ pages? Is there a disturbance in the opening pages that pulls readers forward to know what will happen and/or if the character will succeed? Does this set the proper tone for the rest of the book? (Don't have a Torture Porn opening if you're then going to do a screwball comedy in the rest of the story.)
And most of all, for the author who is beginning to tell the tale: Can YOU live with this character, love her, root for her, put her through the wringer, for up to a year as you write the book? If not, you'd better go back and rework it so that you DO want to spend time with this person. Because if you don't like this character, readers won't, either.
Oscar Wilde Quote of the Day:
"There are men nowadays who cannot distinguish between the truth and the last thing they happen to have read."