The Metro/WinRT XAML ScrollViewer by default allows to zoom in on its contents. That is because its ZoomMode property defaults to “Enabled”. I think in most cases it is actually not the desired behavior and you might want to set ZoomMode to ZoomMode.Disabled. I do understand though that this makes the feature more discoverable and does not hurt much while potentially getting users familiar with the new paradigm of quickly scrolling by zooming that is also displayed in the SemanticZoom control.
For my application I needed to enable users to zoom in on a horizontal StackPanel with a list of buttons so that all the buttons fit on screen. At first I thought I would use the SemanticZoom control, but then I realized I need an actual zoom not a semantic one, so the improved (over WPF/Silverlight) ScrollViewer is an obvious choice.