From Reddit:
The dark haired little girl is all but confirmed as the "new" Sarah Connor. James confirmed a terminator is sent back in time to kill her because she is of great significance in the future. Before that, she's just a normal woman, unaware and involved.
They chose the dark haired girl as a Mexican in Mexico City because they wanted to make the movie relevant by mixing it in with all the immigration stuff going on with US/Mexico
Sarah is a terminator hunter. She's figured out a way to know when they show up, and she's there waiting.
It's R rated!
Arnold is a T-800
Arnold maybe/probably
won't say "I'll be back"
Arnold is the most interesting T-800 you've seen yet, and he has a few funny lines.
The new terminator is named "Rev9"
The whole story takes place in 36 hours
It's not complex, it's Grim, it's Gritty, it's Fast, it's intense, it's very linear.
Cameron describes it as "a white knuckle ride", recapturing the feeling from the first movie
Judgement Day is inevitable. In T2, they kicked the can down the road.
Elaboration I would like to add to the summary:
The tagline "the day after judgment day" is all about that moment the next day in 1997 when it didn't happen, and the realization of being responsible for whatever may now happen as a result of this new future they created.
That new future being the world of today. Cameron stresses that he wants this movie to be relatable to the world we live in. It takes place in the now. And a big reason he came back to the franchise after almost 30 years was because he wanted to tackle the idea that what was science fantasy in 1984 is not as farfetched anymore. AI is coming.
What Sarah stopped is just around the corner again.
He was heavily involved in the story of this one (unlike 3-5) and has also mapped out a whole trilogy roadmap working with Miller, but he values directorial voice and so after handing off the script he let him do his job. He wants the movie to be Tim Miller's.
Most importantly he stressed that their goal is to match the tone of the first two movies, with simplicity, fear, and frantic pace. No complicated timeline shenanigans, and less cheese.
This movie serves to pass the baton to new characters.