https://www.w3.org/Bugs/Public/show_bug.cgi?id=21155 already points out that APIs that allow access to decoded media may not behave in the same way with encrypted media.
We should specify whether there are other changes and where there shall not be changes.
For example http://lists.w3.org/Archives/Public/public-restrictedmedia/2013Apr/0001.html asks whether synchronization between tracks will still be provided in the same way (of course it should be).
Also, it is possible that with hardware media pipelines, CSS transforms may not be fully supported - the media element may be restricted to occupying a rectangular region with top and sides parallel to the top and sides of the screen, respectively. It seems possible that scaling of the video image away from its normal aspect ratio (stretching) may also not be supported.