Simplified version of the application design I'm heading toward (actually in Sinatra, but that doesn't really matter here)

application_controller.rb

class ApplicationController
  # Sharing the setup of processors, since many actions will use them
  #   - thinking of switching this out for a facade
  def trigger_processor(name, listening_controller)
    processor_klass = processors[name]
    processor = processor_klass.new(params)
    processor.add_listener(EventNotifier.new)
    processor.add_listener(listening_controller)
    processor.process(current_user)
  end

  def processors
    { 'new_comment' => CommentProcessor }
  end
  def current_user
    # Returns an object that responds_to?(:can?) so we can authorize
  end
end

comment_controller.rb

class CommentsController < ApplicationController

  def create
    trigger_processor('new_comment', self)
  end

  def new_comment_success(comment)
    # redirect
  end
  def new_comment_error(comment)
    # show error
  end

end

comment_processor.rb

#example class
class CommentProcessor
  def initialize(params)
    @announcer = Announcer.new
    @params = params
  end
  def repo
    CommentsRepo.new
  end 
  def add_listener(listener)
    @announcer.add_listener listener
  end

  def process(user)
    if before_filters(user)
      new_comment = Comment.new(@params)

      # May refactor this to pass announcer on to Comments repo.
      if repo.save(new_comment)
        @announcer.announce('new_comment_success', new_comment)
      else
        @announcer.announce('new_comment_error', new_comment)
      end
    end
  end
 
  def before_filters(user)
    if user.can?(:create_comment)
      true
    else
      @announcer.announce('new_comment_error', new_comment)
      false
    end
  end
end

The main difference here from the original gist is that the processor can announce both success and failures, and those can be handled separately - the notifier may notify social networks only on success.

Not sure to handle the before filter - in certain cases I want to announce authorization failures so they can be logged, so leaving that in the controller seems like it'd have to know too much about about announcing.