Along with being billed as a return to Magic: The Gathering's most well-known world, Dominaria United throws new mechanics into the mix via fresh Legendary Creatures, a spin on Saga cards (which usually task you with meeting certain criteria before getting an affect) that let you skip ahead, an 'enlist' system where creatures combine their powers, and off-color kicker abilities that allow you to upgrade your spells.
In addition, Dominaria United sees the introduction of Standard-legal Jumpstart Boosters. These packs are designed to let players skip deckbuilding and get playing straight away with randomized cards, but you can also add them to your Standard decks.