Chronicles of Prydain/WMG
Taran is actually Cursed with Awesome.
People take great pride in their names in Prydain. Gwydion is able to defeat the Horned King with knowledge of his true name. Achren is a threat to Arawn because she knows his. The fact that Taran does not have a name, that he is an unknown foundling of unknown parentage, is extremely troubling to him and he spends an entire book searching for answers. In the cosmology of the Prydian universe, a person's name is an extremely powerful and important thing.
Gwydion claims that to name something is to have power over it. Yet, Taran's true name is unknown to anyone. This is what allowed him to become what he did. He was not beholden to family or rank or station. All the other characters find themselves bound to their roles. Fflewddur chafes at his role as king and longs for the freedom to be a bard. Eilonwy resents being sent to the Isle of Mona to live up to her role. The people of the Free Commots all know their place in life and do not seek anything more. Taran was not bound by any of this. His lack of a name gave him freedom. He considered not having a name to be a source of great sorrow and frustration, when in fact it made him freer than any man in Prydain.
Adaon's brooch did not have any power at all.
Adaon had visions and dreams because he himself was wise, insightful, and observant. When the brooch passed to Taran, he became wiser and more contemplative because he believed the brooch to be magical. In fact, these traits were inside Taran all along, and in later books they begin to show themselves without the brooch. He did not need the brooch at all, and only lost these abilities because he believed he could not have these talents on his own.
Why did Orddu accept a non-magical trinket in exchange for the Cauldron? All she asked from in exchange was something Taran prized as highly. She knew what the brooch meant to him, and considered it a fair exchange.
Morgant sold his honor for knowledge of the Black Cauldron.
A recurring theme of The Black Cauldron is honor and when to put it aside. Taran at one point must learn to do this himself.
Many people mention that Morgant is a great hero renowned for his strength and bravery. Morgant himself also makes note of Taran's mettle and takes a liking to him, as though he recognized Taran as being not unlike himself. Yet it is revealed that he has become corrupt and wishes to use the Cauldron for himself. He mentions only that he paid a price to the three witches, as Taran did, to learn how the Cauldron worked. Like Taran, he gave up his honor, and in doing so, became a villain.