What makes a plugin (like a module or a block) )Moodle 2.3 compatible? (if you compare it with 2.2 and 2.1)
You'll need to read this and the various links to the upgrade.txt files.... Hopefully, the information is exhaustive!
http://docs.moodle.org/dev/Moodle_2.3_release_notes#For_developers:_API_changes
http://docs.moodle.org/dev/Moodle_2.3_release_notes#For_developers:_API_changes