1) It is the book of mystery. Amen, God reveals Himself though scripture, He doesn't hide in it's pages, but some of what goes on in Genesis is so wonderfully unique (and not, as we'll see). How did Jacob wrestle with God, why do we need to know about Onan (we must need to, or it wouldn't be in there). Is there another book where genealogies and geography are such a key to understanding the overarching story? Is there another book where and old man's bones in a tomb hundreds of miles away from the scene of the action is the cause for such hope.
2) It is the book of beginnings. It is the book that makes sense of everything, both in the Bible and in much of life. Why is that tree there? Because God spoke and it was so. Why is everything, essentially, a mess? Because the serpent spoke and Adam listened. Is there any hope? Yes, lots, as we'll soon see. Beginnings. Of the universe, of man, of the Gospel.
3) It is the book of promises. Right from the chapter 3, we see the proto Gospel, the promise of a serpent crusher. Even though it is these hopes that are crushed time and again, the promise towers over the book. It's not Abel, or Isaac, or Jacob, it's someone much better. Abraham is promised a son, millions of sons, and land. The Genesis promises stretch centuries into the future, they hold the book together. As we read of Isaac's birth, near sacrifice, as we read about the ark it makes us cast our eyes and thoughts upon Jesus.
4) It is the book of us. I'm not sure there's much that happens in the rest of the Bible that doesn't happen first, or is at least promised in Genesis. God being gracious to His people, God judging the sin of His enemies, idol worship, murder, human pride, rampant sin, human failure, sexual perversion, God's purposes ruling over all in spite of man. Brilliant.
5) If it were a TV sitcom, it would be taken off the air. Isn't that true? Especially from Jacob onwards, too much sex, too much violence, no family planning, only one real hero in Joseph. And given that list, it would probably be Christians who complained about it the loudest...