The latest Livestream from the Pantheon team takes a deeper look at the Shaman but also gave players a bit more insight into the Mastery system.
One of the topics the developers touched upon was downranking of spells and abilities, which have been removed from the game, due to it cluttering the spellbook. More info about this will be probably come in the future with new updates to the spellbook.
On the topic of the Mastery system, the devs stated that a Mastery bar will be visible along with your normal XP bar. Mastery XP will be earned alongside XP starting at level one. You will also be getting Mastery points from leveling up, from certain quests and from rare mobs that will net a lot of Mastery XP. Rare items called Mastery Shard can also drop which can be turned into a Mastery Crystal that you can either sell or turn into a Mastery Point.
Mastery Points can be spent on upgrading spells in your spellbook, or you can be spend them on improving your characters in alternate ways.
Mastery points will be earned through various aspects of the game. Since they can be spent on many different things, two characters of the same class and level can end up playing differently depending on how they have spent their points and how much time they have spent on acquiring new point.
Head over to the Pantheon Youtube channel to see the full stream.