Some things that I paraphrased as the chat was happening:
JKR said she doesn’t remember promising anything major from Ginny in DH. She said she thought that Ginny holding her own in battle at the age of 16 proved that she was a powerful witch.
Hermione worked for the Dept. for the Regulation and Control of Magical Creatures before going on to Magical Law, and she did improve the lot of House-Elves.
Luna married later in life than the others; she married a fellow naturalist, the son (or grandson?) of Newt Scamander.
Remus was killed by Dolohov, Tonks by Bellatrix.
Teddy was raised by Andromeda, and had much contact with Harry and his father’s other friends in the Order while he was growing up.
Ginny played for the Holyhead Harpies, then retired to raise her family and write about Quidditch for the Daily Prophet.
Snape’s portrait didn’t appear because he had abandoned his post before dying; JKR thinks Harry would work to make sure it was eventually included, however.
She described the Hufflepuff common room, which sounded rather like a Hobbit hole to me (near the kitchen, cozy, comfy, with underground tunnels and round doors leading to the rooms).
Ron works with George at WWW. No mention of him being an Auror with Harry. Did she change her mind, or does/did Ron have more than one job?
George names his first son after Fred. He never quite gets over losing his twin, but WWW is very successful.
The person who does magic late in life was cut from the series. I guess I was wrong about it being Merope.
Lily did really care about Snape, and might have grown to love him romantically if he hadn’t been so into the Dark Arts.
Teddy Lupin is not a werewolf.
On which murders made which horcruxes: The diary - Moaning Myrtle, the cup - Hepzibah Smith, the locket - a Muggle tramp, Nagini - Bertha Jorkins, the diadem - an Albanian peasant, the ring - Tom Riddle Sr.
Re: Ron and the Deluminator: “Dumbledore understood Ron's importance in the trio. He wasn't the most skilled, or the most intelligent, but he held them together; his humour and his good heart were essential.”
The unspoken thing Hermione smelled in the Amortentia was the smell of Ron’s hair.
When Harry’s children go to Hogwarts, Slytherin is less pure-blooded, but still has a dark reputation.
Harry didn’t pass on the Marauder’s Map to any of the children, but James sneaked it out of his father’s desk one day. However, she phrased her answer with some uncertainty (“I’ve got a feeling…”), so I think I’ll continue to believe fernwithy’s version: that Harry gave it to Teddy Lupin.
Harry lost his ability to speak Parseltongue after Voldemort’s soul was removed from him; he was glad to be rid of it.
Umbridge was imprisoned for her crimes against Muggle-borns.
People did know that Snape and Lily were friends, and that they stopped being friends. James suspected Snape was in love with Lily, and that factored into his treatment of Snape.
On Albus/Minerva: “Not everybody falls in love with everybody else!” Oh, JKR. You know fandom, don’t you?
Scorpius has a lot going against him, but would be an improvement on his father, “whom misfortune has sobered.”
The Resurrection Stone was probably trampled by Centaurs and buried into the forest floor.
Quirrell taught Muggle Studies prior to his one-year stint as DADA professor.
Narcissa was not a full DE and did not have the dark mark, but her views were identical to Lucius’s until Voldemort turned against their family.
Winky dried out a bit and helped fight the DE’s with the other Hogwarts Elves.
Marrietta’s spots did eventually fade, though they left some scars.
Snape never showed his Patronus to Order members. His role as spy meant that it was not generally safe for him to use that method of communicating, anyway. This begs the question: How did he contact the Order at the end of OotP?
Snape went to 12GP immediately after Dumbledore’s death, before Moody set up the enchantments against him.
The reason Harry sees King’s Cross while he is “dead” is that he’s always seen it as a gateway between two worlds. Also, “the name works rather well.” <-- point in favor of the argument that the chapter title alludes to Christ, I think.
She said over 120,000 questions were submitted, and that she will try to address more of them on her website, so keep an eye out for updates.
ETA: Full transcript here. (Beware of SPOILERDRAGONS, as well as C & P errors.)