Funny. I'm experiencing the opposite reaction. The iPhone 6 was, undoubtedly, the best iPhone ever released, feature wise. However, I also think it was the "worst." I was so pleased with my iPhone 4 and then, to a lesser extent, my 5. They certainly had less features than the 6, right? But there was something about them, particularly the 4, that was "special."
I didn't really get that from the 6. iOS 8 was buggy, the phone itself was awkward to hold (too slippery, too light almost). The camera and the antenna were eye sores of the nth degree. The Pluses had a penchant for bending. Apple Pay was the only major feature that was "special," and that was sort of blunted by slow adoption issues.
The 6s, however, I adore. The camera and antenna are still atrocious looking. However, I really Love the new materials and weightiness of it. It's less slippery and feels more substantial to hold on to. 3D Touch looks like more than a gimmick, and looks like it should signal a real paradigm shift. Live Photos are silly and gimmicky but I can't help but geek out over them and think about Harry Potter. iOS 9 fixes a lot (although not all) of the odd design/utility bugs. The improved "Selfie" camera is actually really nice and, as a millennial living in the age of Snapchat, surprisingly useful and appreciated.
Everything feels more refined, and even though the phones look similar, I genuinely think the 6s has superior design sensibilities, as well as awesome and exciting new features. It's also a joy to hold. I just wish they had fixed the camera protrusion and antenna lines. Other than those two grievances, I think I'm enjoying this phone more than any other, save the iPhone 4.