Apart from the processor and camera in pixel ranks
Iphone 5 to 5s => touch id (major, although not the first)
Iphone 5s to 6 / +=> phone design and larger screen (major in design, not the first as well)
Iphone 6/+ to 6s/+ => force touch (this is the first)
6s/+ to 7/+ => water resistant ip67 (major but not the first), no headphone jack (if we call this improvement) and double camera for plus version
7:+ to 8/=> glass cover design (nice), wireless charging (hm ...... no comment)
X => well, it’s just simply different with previous iphone
Not the first in every aspect for sure (and that’s make people who hopes for something new, not too excited, including me), but what makes me stay is the ios (stable, fast, secure, frequent update, and can sustain until 5 maybe years update )