Mera literally making absolute mincemeat out of her enemies is downright scary awesome.
Queen Atlanna seeing through Orm and Black Manta's attempt into making her think the surface world had attacked Atlantis. While it doesn't save her in the end, it shows she wasn't stupid.
Orm taking down the entire Justice League with the power of the trident shows just how serious a threat he is (or rather the trident), making even Superman bleed not once but twice on separate encounters. Seeing Arthur finally punch his lights out with a blow strong enough to break his helmet is very satisfying.