The needs and features for the ideal plugin framework include thoughts about runtime manageability, complexity and learning curve, integration effort, logging, memory footprint, startup time, bloatedness, testability, level of documentation and maturity. That's a lot of stuff to verify and it requires a smart approach to get it done.
Mats uses Langur, one of the applications currently using the Atlassian plugin framework, to do his research. His first smart move was to make the plugin framework pluggable. This was achieved by adding an abstracting layer for the plugin framework. With this layer the application can be started with (or even without) any plugin framework implementing the desired set of requirements. Of course the Atlassian framework was plugged back first. This step proved that the Atlassian plugin framework does offer a lot of functionality, but scores are down on the bloatedness and (lack of) documentation.
Currently Mats is exploring other frameworks, among them Impala, Java Simple Plugin Framework and Apache Karaf. If you have pointers for him or questions about his progress let him know!