I would say that one of most significant things Odysseus learns on his journey home from Troy is who he really is: he rediscovers who Odysseus is and what it means to be Odysseus.
Throughout his adventures, Odysseus constantly encounters questions and challenges about his identity. At times, he is tempted to forget who he is (as in the case of his encounter with the Lotus-Eaters). At times, Odysseus reduces himself to a person without an identiy, as when he tells the Cyclops that his name is Nobody. Sometimes, Odysseus disguises who he is, as when he returns to Ithaca and disguises himself as a beggar. On many occasions, Odysseus tells stories, pretending to be someone else, and tells other people (including his own wife and father) that he has seen Odysseus.
Another thing Odysseus learns is to be cautious about revealing who he is until just the right moment. His revelation of his name to the Cyclops at the wrong time allows the Cyclops to be able to curse Odysseus.
Finally, Odysseus learns the meaning of "home." Throughout the Odyssey, the hero sleeps in many beds. He sleeps with goddesses (Calypso and Circe), he sleeps in caves, he sleeps in leaves after he washes ashore on Phaeacia. He does not truly reach his home, though, until he reveals the secret of the bed he shares with Penelope. Once Odysseus tells Penelope the "true secret of our marriage bed", then he has finally returned to the home in which a true Greek belongs.