Can't Opal earth, metal and air bend? I never seen her metal bend.......or my memory is hazy.
Opal didn't have any bending before Harmonic Convergence. She then gained the ability to air bend. Not every son or daughter of a bender is a bender (just like Bumi in the first two seasons).
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Anyway, I found the ending quite disappointing overall. Kuvira turned out to be a wet blanket and Korra was too restrained. When they got inside the mech Korra should have gone into the Avatar state and then wrecked shit up in the cockpit. No holding back; no trying to talk Kuvira down; she should have blasted in there and beaten the ever living shit out of Kuvira, and then presented her unconscious body to her army and ordered them to stand down and disperse. We know she can take Kuvira in the Avatar State, from when they battled and she briefly had the upper hand outside of Zaofu. And then in the spirit forest Kuvira suddenly has a change of heart (bearing in mind this is almost immediately after trying to murder her fiancé in order to kill Team Avatar) because Korra talks her down?! She should have at the very least taken away Kuvira's bending away, because she was insane and too dangerous and many times directly tried to murder Korra and her friends. Yeah... no. Not buying any of that. Plus, there was no resolution as to the 'Dark Korra' plotline that was the very first thing that happened to her this season. I appreciate maybe it was manifested from her PTSD and lack of belief in herself, but at the very least she should have had a confrontation with the dark spirit, even if she faced it down in the Spirit World and said something cheesy like "I'm not afraid of you any more!" I guess that plotline was resolved when Zaheer (
) showed her how to get back to the Spirit World, but again there was no conclusion; it was just kind of left hanging.
As usual, the Beifongs were my favourite characters. Su is so goddamn awesome and I'd have loved her to have been the main character. I'd gladly see a series based around Su and Lin; I really like the way the twins sons act as one and Bolin could be in there as well as Opal's hubby/fiancé. And then Toph could show up periodically to offer her kind of tough love advice, but without really getting involved in the action. Operation Beifong was easily my favourite episode and I think that fight between Su and Kuvira is one of my favourite battles in all of ATLA/TLOK. It's short, but so sweet.
Overall I'd rate the seasons in order of quality:
#1 - Book 3: Change
#2 - Book 1: Air
#3 - Book 4: Balance
#4 - Book 2: Spirits
All ATLA seasons come above these, easily. I'd need to re-watch all of that before ranking those in order.