Sauron's "enter the battlefield" effect is a classic Threaten effect, taking control of an opponent's creature, untapping it, and giving it haste until end of turn
(It can't be blocked except by two or more creatures.) If you controlled a creature with power 4 or greater when you declared Courageous Goblin as an attacker, it doesn't matter whether you still control one as its ability resolves
Currently, people are underestimating the value of starting with an extra card in hand each game, but with the set being released shortly, I expect people will wake up to the mechanic's absurd power soon
However, one-mana tutors that can search up a wide array of cards are some of the most efficient out there, and like Seedborn Muse This card causes you to take what is essentially, when built properly, three additional turns worth of effects each turn cycle
Baral, Chief of Compliance and Archmage of Runes are premier counterspell payoffs