3 things. Battery life. Camera. Display.
For me, battery life has been unbelievable. It's the first smartphone I've had where I can easily get through a full day of mostly moderate-to-heavy usage and still have anything left, let alone the 20/30 percent I'm getting at the end of the day.
The OIS in the camera is a huge improvement for me. I have horribly shaky hands when trying to take a picture (the more I concentrate, the more I shake - it's psychological, not neurological). The Plus has made it so that I'm getting less jagged shots and more acceptable photos per batch.
The display is amazing. I'm pretty heavily nearsighted and for me going from a 6 to a 6S Plus was a big change for me. I really can see more clearly now things on the screen and I squint way less, and so therefore I'm having less tension headaches, lol.
All in all, a huge jump. The cool S features aside, going from regular to plus was the best choice I could have ever made.