Is it possible to have a "useful" button that only counts for the post, not the poster? Perhaps a button that would add a thanks and a useful?

Then sort on useful posts. Just because someone posts a popular opinion/funny joke, the post may get many thanks, but not be useful to the pupose of the thread. As people read through the thread, it is easy to mark the useful posts and or thank posts. If someone wants a quick overview, scanning only useful posts would be better than scanning the thanked posts and filtering out the joke, etc that got thanked.

*edit* to clarify, in a "I have problem x" threads. Many people help the op and post suggestions on how to solve the problem (many which may not help). These posts get thanked (rightfully so). Other posts are thanked for confirming the problem etc. Some of these threads get quite long, and 3/4 of the posts have thanks. When a solution is found, it can be marked useful, as well as confirmation of the solution, or clarifications. Then filtering by useful, you get the problem(op) and the solution(useful) without all the trial and error inbetween.