I found out today, via an error report from the field no less, that one of my apps which uses some of the Extensibility hooks (aka App Connect) that were released as part of WP7 Mango doesn’t work on WP8. Upon investigating, it appears that when MS recompiled our apps to run on WP8 devices, it didn’t properly bring forward these specifications in the App Manifest, Extras.xml, and other locations which make these work.
Right now, I’m working on having two separate WP projects to solve this issue, which share the files via Add Link in Visual Studio, but are compiled against WP7 and WP8 in and of themselves. I’ll let you know how it turns out.