In short, you want to balance the mana value of your spells to ensure you have effective spells to cast throughout the game
Horsemanship is a new mechanic introduced this set that basically works like flying, where you can only block a horse rider with another horse rider
Card draw plus mana refund makes Teferi absurdly efficient
Smoke Spirits' Aid Wizards really needs to slow down on things that make a bajillion treasures, because it's contributing to the Commander format getting faster and faster
The biggest thing to learn are which abilities you can trigger so here are a few great examples of allies with text that includes when, whenever, and at: Katara, Bending Prodigy, South Pole Voyager, Ondu Cleric, Earth Kingdom General, and Hero of Goma Fada