I have serious doubts that Apple's 3D Touch will be a huge "success" with users unless there is a hint that touchable components in fact do support it. I am not a UX expert, so the points that I'll bring up are merely based on the little knowledge I have acquired as a design enthusiast and UX aficionado.
1) Recognition rather than recall
In UX this has become a rule that UX experts abide by religiously, so why should it be different with Apple's 3D Touch ? Why/How should users recall that an icon supports 3D Touch ? More importantly, how should they recall which actions are supported by the opening menu? The more intuitive way is to tap an image to enlarge it, right ?
The counter argument is the case of context menus on desktops we know and love (right click components to show actions) such as copy & pasting text. The thing is, that those context menus often do have a predefined set of actions that are universally known (and recalled) by users (e.g. Copy, Paste, Settings), which will probably not be as consistent in iOS apps.
Suggestion towards recognition:
Instead of guessing whether a visual component supports 3D Touch:
Steer towards recognition -> Add a visual guide that the component supports it: (The hint icon looks terrible, I know, this is just a quick draw-up for illustrating my point)
Of course this wouldn't make much sense in the case illustrated (as all app icons support 3D Touch by default), but in general (custom built apps that support 3D Touch in their interface).
2) Significant delay in interaction -> hampering UX
From what could be seen in the demos, enlarging an image takes 50ms (?) or more, why would you even bother to hold down to enlarge an image when you can just enlarge it with a tap, which is probably much faster anyway ?
I am looking forward to hearing what experts (or more experienced people) on here think about this.