Make sure your characters learn something by the end of the book.
Make sure the reader is rooting for them.
Make sure they have something at stake in the book - a new business they want to succeed, a character trait they want to overcome, an ambition they want to achieve etc.
Make sure they have pasts and futures, talking or thinking about hopes, fears, dreams. Make sure they do things throughout the book to help them achieve their goals.
Make sure thehero and heroine are likeable. This doesn't mean they have to be perfect, but the reader has to care about them. Make sure they have have flaws, but make them understandable flaws and also basically likeable flaws. Self pity isn't a likeable flaw, for example. Rashness is.
Make sure they react in believable ways.
Make sure you would like your hero and heroine in real life.
Make sure they try to solve their problems rather than just moaning.
Make sure we see them in a range of situations so we see a range of emotions.
Make sure they're not Too Stupid To Live. A heroine who takes her cheating boyfriend back time after time is too stupid to live, for example.
Make sure you don't have too many minor characters and make sure you don't let them run away with the book.
Make sure your hero and heroine are together in most scenes and have meaningful interactions that engage their emotions.