5. A clean future. Why is Jai Courtney a clean, hairless, ripped Ken doll instead of the dirty, scarred Kyle Reese from T1? He's nearly as ripped as young Arnie and basically looks like a terminator; the contrast between the two time travelers was a key to the first movie's sense of desperation.
4. John Connor's powerset isn't clearly defined. One of the things that makes terminators so scary is that you know from the start all the terrible things they can do and they will do all of them to kill you from minute one. John Connor holds back, and that actually makes scenes less scary early on - and later scenes feel a bit like a cheat.
3. The music. While there were some classic riffs in the soundtrack, most of the music was "generic action/adventure."
2. The helicopter chase. This was 100% CGI, ridiculous, and felt out of place from the rest of the movie. There wasn't even a cool third arm thing like in T2 (if you don't remember this from T2, go back and watch it, paying attention to the T1000 in the helicopter cockpit).
1. Jai Courtney. His acting was literally the worst. If you had put almost any other human in the role of Kyle Reese, this movie would have been 100x better.
5. Khaleesi. Let's face it, lots of people are going to see this movie to see the mother of dragons fight robots. She does a good job of infusing the movie with some actual emotion.
4. The time travel. Yes, it's wacky, but it manages to balance both determinist and "no fate" ideas in a way that makes-sense-as-you-go without giving any real answers. Time travel not making sense is actually key to the Terminator franchise as the conflict between fixed and mutable theory is what gives them their sense of real danger.
3. Arnie. He's still got some acting chops, even when playing a robot.
2. The humor. T2 had a lot of lighter moments, despite being about murderous robots from the future. T:G remembers this, and abandoning the grimdark approach of Terminator Salvation is a good thing.
1. Recreating scenes from T1. CGI Arnie actually reminds you a lot of the animatronic Arnie from T1 in a way that works really well.