Before explaining the new plugin framework, let's define what problems we faced with the plugin subsystem in MuseScore 1.2.
One problem is the binding to the qt library. The code to enable this is very huge and consists of > 1100 files with > 297000 lines of code. This is not maintainable for the MuseScore team. It seems that qt does not maintain this anymore and it will not be available for the next major Qt version.