Druckmann explains the original pitch in the first post of this thread:
http://www.neogaf.com/forum/showthread.php?t=645363
What that post leaves out is the original plot climax and some details leading up to that.
Joel and Ellie are in a truck, and Ellie wakes up. He tells Ellie that it's too late for a cure, and they're going to move on. Then the truck is rammed by one of Tess' men, and Joel and Ellie take refuge in a nearby mansion. Joel tells Ellie to escape the mansion and steal one of the gang's cars and he'll catch up with her in the next town. She does so, and Joel is stuck fighting off the gang. He eventually fails, which leads to the torture scene depicted in the picture.
Tess decides to torture Joel in order to find out where Ellie is, she intends to kill her as payback for Tess' brother dying. After Tess figures out Joel won't break, she moves to shoot him in the head. A shot fires, cut to black.
Ellie is driving towards the town in her stolen car, but stops. She figures out that Joel won't be coming, that he's going to die in that mansion if she doesn't help him. After driving back the player takes control of her again, in order to rescue him. After some gameplay, the cutscene goes back to Tess about to kill Joel, but now you see that Ellie was the one who fired, killing Tess. And then they have their happy ending in San Francisco, with Ellie fully buying into the lie.
They had to kill that ending because the motivations for all the characters didn't work, and required changes to almost all the principle characters. Tess no longer had a brother who died, Tess was bitten instead of Marlene as originally envisioned, and the climax of the game would take place at the hospital instead of a mansion.
The second time around they still kept the happy San Francisco ending, but again, they dropped that because it didn't feel right, the character arc of Ellie felt incomplete to Neil. So it became the ending that everybody's seen by now. It didn't focus test 100%, but the responses they got felt like the right kind of responses.
Druckmann's little secret is that he intended for Ellie to be the strongest character in the game, stronger even than Joel. From being surrounded by parental figures to growing into an individual that is more than capable of handling herself, that was her character arc. In the end, Joel needed Ellie more than she needed him.
He wanted a character, a role model, that was unlike any female character currently in the industry (which is a whole bunch of other threads that we can talk about some other time). His final slide of the lecture was a bunch of photos of Ellie cosplay, which gives him hope that he has succeeded.
This is mostly from memory, apologies to Mr. Druckmann if I left out anything or remembered it wrong.
Some random tidbits:
Ellie was going to get a dog around the middle of the game, but they couldn't make it work gameplay-wise so they had to drop it.
Joel was initially written to be more closed and brooding, but Troy Baker's performance brought an intensity to the character that they worked into the game. Ellie was written as much more passive during the initial hunter attack after the crash in Pittsburg. Ashley Johnson noted that it wasn't in her character to just do nothing while these guys attacked Joel, and the stuntman was totally cool with Ashley hitting him and struggling on the next take.
Nolan North wasn't cast as David right away. The character's proto-form was "the Cannibal King" as he was the leader of the band of cannibals, and was one of the few characters that was able to have motivations beyond simple survival - taking actions for pleasure instead of necessity. North wasn't part of the principal cast yet and wanted to be involved, even if it was just a bit part. They obviously gave him a meatier character.
