Directly inspired by the Warlock tome that turned their fire spells green, Devotions are a way to change the appearance of your class abilities. Here are some examples.
Priest: Devotion to the Light - Changes the appearances of Shadow Spells to look more holier.
Priest: Devotion to the Void - Changes the appearances of Light Spells to look more shadowy.
Priest: Devotion to Elune - Light Spells are White colored, Shadow Spells are Dark Moon colored.
Paladin: Blood Knight Oath - Replaces all the Hammers with something more Blood-Elf related. Like the phoenix symbol, or a sword.
Paladin: Sunwalker Oath - Similar to Blood Knight Oath, but with more tauren features. Like BoJ being more earthlike, Divine Storm showing Sun symbols instead of Hammers. And Hammer of Justice has a big totem instead of a hammer.
Mage: Pyromancy - All Arcane and Frost Spells take on a more fiery appearance.
Mage: Cyromancy- Similar effect, but with Frost.
Mage: Arcane Affinity - Ditto, but with Arcane.
Mage: Frostfire Mastery - All specs take on a more frostfire appearance, including arcane.
Warlock: Necromancy - Warlock demons are undead instead. Imp becomes Skeleton Mage, Succubus becomes Banshee, Felhound becomes Ghoul, Voidspawn becomes Abomination, Felguard becomes Revenant, which is like a lesser Death Knight. Idk. Rest of the spell appear more necromantic.
Any other ideas of Devotions would be welcome.