When 3D Touch was presented I thought it was a small breakthrough.
One of the major issues in mobile UI design, even in nowadays implementations, is (IMHO) that one is forced to go through zillions of taps just to reach that particular command that would, in a desktop-like interface, be promptly available in a contextual menu. And boom, 3D Touch provided the possibility to have just that, alongside other actions 'hidden' in the 'third dimension' of your screen.
When I went from an iPhone 6 (no 3DT) to a 7, I all but confirmed that first impression: I absolutely loved the feature where it was well put at work (the keyboard, the shortcuts from the app icons in the home screen). I also noticed, on the other hand, that it was
1) impossible to choose whether to use it for the said contextual menu or for a force-touch like preview of visual contents. I find the latter not only rather useless, but actually difficult to use, especially while walking or moving, given the two levels of pressures switching between the preview and the full opening of the content. In practice, I always end up opening the content, which is equivalent to tapping on it making the 3DT presence irrelevant.
2) even worse, there is no hint whatsoever in the interface to try and use pressure on an element, resulting in most users completely unaware the feature even exists.
In my opinion these are major flaws that explain why 3D Touch never caught on neither among users nor app developers. And like several people here already pointed out, it's yet another example of how the post-6 (i.e. post Jobs) iterations of iOS were never really spot-on like the previous ones as per the interface, and in general of how Apple seems to be increasingly overlooking the software/hardware integration in its own products.
