The app shortcuts are great, like you mentioned.
The cursor idea is genius.
The potential for gaming is obviously there, it's a whole other button or two that you can press anywhere on the display.
The ability to peak into apps/parts of an app/links/etc... (I like it for links more) is pretty damn useful. The "thumbnails" that pop are are interactable as well so you can get things done quicker.
A way of getting around the OS without any buttons (hard or soft) is paved with 3D touch. in iOS9 there's already a way to get to the app switcher by pressing down hard on the edge. Not sure if there's a way to get to the homescreen from any app but I'm sure it can't be terribly far away.
etc...and this is just first time at bat stuff. A year or two from now I'm sure it'll be doing more.
Yeah the peak feature highlights the potential in the 3D Touch feature. I guess you could argue that it currently serves as a more useful right click but the implementation is what matters. The feature is responsive, fast, and has never failed or accidentally activated in the month that I've owned the 6S. I feel these factors are what separate crap that people never use again to features that are carried forward to successive smartphone models.
With regards to the comments that its basically a long press, the long press just isn't as fast as 3D Touch and I don't see how it can emulate additional/secondary functionality through additional force. The peak + swipe up to hold window and display contextual menu + harder press to bring the window into full focus is perfect for a skim email reader and web browser like me.