When I first saw the Windows 8 Semantic Zoom feature, it struck me as another touch-centric feature that looked flashy but wasn't actually useful. It's only when you see it in an actual app that it makes sense.
The standard demonstration of Windows 8 Semantic Zoom -- I've experienced it at least twice from Microsoft staff -- involves showing how you can shrink the Modern start screen so that your icons are tiny. It looks flashy, but who wants icons that small? The only reason I could imagine using it was to rename groups, and only because that option is invisible otherwise.
The light bulb moment came during the demo of applications developed at the AppFest event at TechEd 2012 last week. The developers of the Feedback Loop app showed how semantic zoom could easily move from a calendar view of times to sessions at that time. That actually looked useful and represents a genuine 'semantic' change (showing meaningful information) rather than just making stuff smaller.
It's not a hard thing for developers to do (as the Feedback Loop team noted, it takes just two lines of code). I don't think every app needs it, but in the right context it's really effective.