Otto, Audrey Capital Tech Ninja and regular contributor at WordPress.org, has published a blog post explaining the difference between WordPress actions and filters. I just hope folks start listening to him, for his sake:
So plugin authors, please, please, I’m begging you, learn this lesson well.
Filters are supposed to filter. Actions are supposed to take action.
When you mix the two up, then you cause pain for the rest of the world trying to interact with your code. My desk is starting to get covered in dents from me repeatedly banging my head into it because of this.