Pressing hard on the home screen icon reveals Quick Actions
One of the features that stand out on the iPhone 6S series is the 3D Touch. It's something you can immediately notice when you use the device for the first time because many of the stock apps take advantage of it. Basically, the phone's screen now knows how hard you push your finger against it, and translates it into triggering different functions.
In the home screen, a light touch on the app icon still launches it. But when you push it a bit harder, Quick Actions menu is triggered for the supported apps, letting you do get into various tasks even before the app is actually launched. It's something that I didn't know I wanted, but I would definitely miss it if it were gone. I'd be slightly frustrated to use older generation iPhones now thanks to this, sort of like how I feel about the lack of Touch ID on iPhone 5 and older. As it's the most obvious way that an app can be extended for 6S series, many of the 3rd party apps are now getting updated to enable this.
Here's a comparison video I made to show how 3D Touch lets you do more. On the left is an iPhone 6 Plus, and on the right is a 6S Plus. When I push the Photos app hard, 6S Plus shows the Quick Actions whereas 6 Plus only recognizes the long press and wants to know if I want to delete any apps.
Once into the app, pressing hard on a photo in 6S Plus lets me "peek", i.e. preview the photo, and if I push even harder, I "pop" into the photo and it's loaded on the screen. A light tapping feedback happens as I do this due to the Taptic Engine. On 6 Plus, nothing happens. This peek/pop interaction can be seen in many other stock apps, so I'm having fun trying to push the screen a lot to see there's anything more I can do.
3D Touch on iPhone 6S Plus & Force Touch on Apple Watch
Now, this sort of interaction isn't particularly new to me. It was shown off on Apple Watch first in the name of Force Touch. The two serve a similar purpose - "tapping" and "pushing" are recognized as different gestures and the appropriate interactions are made available. This is especially useful on Apple Watch due to limited screen space, but I think Apple applied the idea quite nicely to the iPhones as well.
But there's more depth to 3D Touch in that it recognizes several different levels of the push, good enough to make painting apps recognize the varying pressure of the "brush". In the least, a lighter push equals "peek" and a harder push equals "pop". Apple Watch doesn't have this nuance. I think this is why Apple decided to call the technology differently, despite sharing the same roots.
Another thing to note is that I applied a 0.4mm thick screen protector on the 6S Plus. Having this one did not affect the functionality of the 3D Touch. Perhaps this was because the protector was not made of glass despite being about four times as thick as the regular ones.
I'm gathering up and fixing all the iPhones I bought over the years for a nice comparison. More to come.