The style, the atmosphere, the beautiful writing, the indepth characterisation, the wonderful amount of detail... Like the details in the house Lily envisions, for istance, with Petunia's sandles and soft porn that Lily never knew was there.
There's just so much I loved about this, but especially the interactions between the characters and the way Lily interacts with all of them as the narrator, from the perfect dialogue between Snape and Harry to a shy Regulua taking a shine to Tonks and The Grindlewald Effect on Albus. And how Lily decribes them all, even Tom, so that even when she's hating someone she does it, like everything, with passion. I can this woman spurning the young Snape for not doing anything, not showing passion, and becoming baby Tom's staunch defender, and prefering Jame's being anything by complacement towards her. The way she knows Severus, what his movements, like hiding behind his hair, mean. How she sees beauty within things.
The way everything fits so well into the spaces canon left behind...
I ramble, but I just have so much love for this. Only without the 'just'. ;o)