Say, for instance, I configure a support section in my forum, where people can thank others for useful answers, leading to my users "competing" with one another for useful answers to questions. We also have an Achievements category, where people can post when they reach a certain level or passed a certain milestone. I currently run a modified version of the "Post thanks" hack to do this, so others can congratulate the guy/gal for their achievement, without posting little more than a "congratulations!" in a reply. Last example could be a bug reporting category, where one could indicate they also experience a particular bug, leading to more efficient categorizing or prioritizing of bug fixing.
If you want to make it totally awesome, implement it in such a way that per button, we can choose to include or exclude usergroups. The "Post thanks" mod only offers the exclude option, leading to a situation where every time I add a new forum, I have to add it to the list. I'd prefer being able to just restrict the usage to a limited number of forums or a single category, and never having to look back at that ever after