[nacs]

Long-press has existed on iOS for a long time as well (deleting apps from your homescreen is an example).

"3d touch" works and feels different because of the feedback you get via vibrations and the fact that it feels like you're actually pushing the whole screen down. It's hard to explain but feels natural when you use it.

[guelo]

Feedback doesn't improve discoverability. Apple will try to guide developers as to when and where they should implement the gesture. But not all developers will do it consistently so users will always be guessing if the gesture will do anything.

[happyscrappy]

Apple is fairly strict on developers when integrating new features and the gesture is simple with instant feedback.