By special request, here's a super-boring post on the ForumNG 'features' plugin infrastructure! (Not a developer? Look away now.)
It's very simple: you stick standard Moodle subplugins in the 'feature' folder. Each one must have a class which extends either mod_forumng_discussion_feature or mod_forumng_discussion_list_feature (which themselves extend mod_forumng_feature).
The difference is that 'discussion features' will be placed at the bottom of the discussion page, and 'discussion list features' just below the list of discussions on the main forum view.php. Discussion features are passed a $discussion parameter for a bunch of things, and discussion list features are passed a $forum parameter.
The classes for this basically have three methods that probably need implementing: get_order (returns an integer indicating the order relative to other features), should_display (default returns true if you have moderator permission) and display, which returns some HTML.
This doesn't really add up to much; it just means you can put a button (or pair of buttons, or dropdown, or whatever) in the area below the list of discussions or at bottom of a discussion.
Then you can put the code for handling actions in PHP scripts inside your feature.
There's a catch here, which is - well, let's take the 'delete discussion' feature as an example. So there's a 'delete' discussion feature and it provides the button you use to delete (or undelete) a discussion, and the script that runs when you click on it with the 'are you sure' prompt, and it checks permissions and stuff - but it doesn't really do the actual work for deleting a discussion because that clearly ought to be, and is, part of the forum back-end in the mod_forumng_discussion class. (Note: If you want to look at the code for a forum feature, the delete one might be a pretty good place to start.)
That's also true for a lot of the built-in features: the 'feature' architecture actually only includes the UI, and the real back-end work is done inside the core class, because it sort of has to be that way.
But it does still allow some degree of separation.
One of the reasons I wanted it is that ForumNG at the OU includes this totally useless 'show who read this discussion' function. Supposedly this shows you who has read a discussion and when, but in fact it's impossible to tell. So the list, though sort of correct, is basically a lie. I had to do it because they made me. But I didn't want to include that in the public version; no reason to spread our bad practice around the world. The 'features' system meant I could just exclude that one folder from the distribution without changing anything else.
Note: An obvious extension would be to add mod_forumng_post_feature class but we did not find a need for this yet - possibly because the 'post selector' approach (where you click a button at discussion level, but then can select one or more posts with checkboxes) is more flexible.