So in a way "Amends" was the midpoint, at least in terms of how things aired in the regular season.
But more importantly, it marks Angel's transition as a character. Up to his transformation back to Angelus, making amends for his past was, well, not really an issue. He wasn't a bad guy, by any means, but he wasn't really a hero, either. Mostly he just lurked around and fed Buffy information, occasionally backed her up in a fight.
He was one of the good guys, certainly, but he was still defining himself by his relationship to Buffy,and when he died and came back, he didn't even have that to work from. Frankly, the two of them had doomed stamped across their forehead the entire season.
With "Amends," we get a sense that there's some higher purpose for Angel--the First wants him evil or dead echoes of the Shanshu Prophecy and Wolfram & Hart's desire to have him on their side in the Apocalypse.
There was a little destiny tied up with Acathala, certainly, but really Angel had always been second-fiddle in the higher-calling orchestra. But with Amends, we have the embodiement of evil acting directly against him, and some force or another manipulating the very weather to save him. And we never really learn who, do we? Who made the snow, and who returned Angel from Hell? The First claims the former, but is suspect--although we DO see the foreshadowing there of Angel's role in the Apocalypse. But the snow? No. Jasmine perhaps, as part of her long-term plan to return? Wolfram & Hart, protecting a future investment? God? It's a mystery.
Angel's totally at loose ends in BtVS Season Three, and can no longer define himself by his relationship, even when they're back together. It's too problematic, it's too painful. He really only sticks around because of the danger of the Mayor.
Of course, when he leaves, he's still a wreck, brooding in the dark, saving people but looking a little too long at the necks of the people he saves. It's not