We have a custom DefaultInitializer registered for FrameworkElement. It does nothing special - in most cases it does nothing. This problem can be reproduced with the completely empty implementation of DefaultInitializer as well.
When I drop a standard control (Button, CheckBox) on the design surface (on a Grid, for example), it does not get its Content property assigned. Once I remove my empty DefaultInitializer it starts working again.
This problem seems to appear after VS2012 Update 2 (works fine in VS2010 and was working fine before update 2, but I'm not 100% sure about this).