It's really nice to see such a strong card get a reprint in Standard format.
Leyline of Sanctity has been the go-to card against Burn decks in Modern for many years. Now it looks like Wizards of the Coast decided to give the same protection to Satndard players against all the mono-red aggro decks.
Taking into account the new London Mulligan rule, players can aggressively mulligan for Leyline of Sanctity and play it on turn one without paying its mana cost.
It has always been a sideboard card, but against red aggro decks and blue-black discard lists, this enchantment will easily become the most valuable sideboard card in Standard.