When I dragged the clip to its maximum length I did feel a little bump. Without looking at the timeline and looking at the viewer, I could 'feel' the end of the clip.
This feature presages the ability for UI pixels to be 'bumpy' - for user to feel the texture of application UIs without having to look at where the cursor is. This means that seemingly textured software keyboards and control layouts will be able to be implemented on future trackpads, iPhones and iPads.
What I realized in using extensions is how necessary last year's redesign of iOS was. Imagine if Apple didn't ship a new design with iOS 7: today, we'd have sheets of stitched leather or shiny metal on top of apps that look like agendas or little robots. The cohesiveness and subdued style that iOS 7 brought with its precise structure and hierarchy allows extensions to integrate nicely with apps, feeling like extra actions rather than eerily realistic objects.
Assuming Apple's plan is to let users feel the interface on all their devices going forward, can you imagine doing that with UIs that have rugged leather, plastic buttons, and other physical textures close to each other? If that's the case, I'd wager that Apple's focus on clarity also applies to tactile feedback for on-screen elements.