John Connor manages to wipe out Skynet's main base and apparently primary factory in one night and almost by mistake. While at the same time proving that his conspiracy theory was absolutely correct. While saving his teenage father. While resuscitating Marcus. While not dying from a massive chest wound. Just John in general.
He gets a smaller one when he first appears, landing a helicopter on a Terminator and shooting it twice in the head.
Let's not forget how fucking ballsy John is. En route to Tech-Comm base, John decides that He wants to meet up with Resistance Command, to demand answers to what had happened earlier in the day. After being denied his request for a meet-up, John tells the Osprey pilots taking him back to base, to open up the rear-loading door, and to tell Command to send divers to pick him up. He then proceeds to jump out of the Osprey, into a fucking swell, hundreds of feet in the air. This is John Connor we're talking about. He's Human, military training aside. He's not even augmented like Captain America, and He's jumping out of a fucking Osprey in the middle of a goddamn storm, into hostile waters; from hundreds of feet up. That's ballsy.
The sudden appearance of the T-800, which happens to look exactly like Arnold in 1984. Also notable in that this is the first time in the film that the iconic Terminator theme actually plays. Doubles as HSQ.
The opening scene is awesome as well. Not only does it hammer in the fact that the war against the machines has begun, but after seeing a lot of scenes of the future from the previous films where everything is dark and hopeless, as if humanity is fighting a losing war, it's just epic and blood-pumping to see that humanity is able to kick ass and take names.
Teen Kyle Reese and Star, a little girl, taking out a terminator by themselves. And apparently being the only members of the Los Angeles Resistance as well.
Marcus being told that he is a Skynet program designed to infiltrate the resistance. His creator tells him that he will not be the hero of the story and that he cannot save John Connor. Marcus' response? "Watch me."
Then he throws a chair right at the monitor Steve Wilkos-style, leaving it into thousands of bits.
John talking out one of the Terminator bikes and becoming a Badass Biker.
The origin of John's facial scar: He was clawed by a T-800 endoskeleton that had just gotten molten metal poured over it. That is about the most metal (no pun intended) way you could think of for how he got that scar.
All of the practical effects in the movie. All of which were meant as a tribute to the original terminator movies and Stan Winston himself.