First, let me second Dan's request for a "view entire thread" button in each post, and the oft-cited request for an "edit" feature. Plus, while I'm typing this I'm reminded of the need for the entire thread to be displayed while in "respond to this post" mode so I don't always have to have two windows open to refer back to the thread contents. All of these are fairly common with online forums and would enhance the participant's experience.
But my biggest wish is for this simple and logical tool: I would greatly appreciate if a past thread would revert to the front when a new post is added. Then, rather than repeat the same discussions, questions and replies, we could add new info or questions to the relevant older post and not lose our previous conversations. Isn't that the purpose of a "thread" after all