On the first post of every thread, to the upper right there are 3 fields:
Thread Tools / Search this Thread / Rating
If you click on "Rating", you'll get a pull down menu that will allow you to vote for the thread in one star increments, from terrible, bad, average, good, and excellent.
Some thought enough of this thread to vote it as excellent, while others thought otherwise and gave the thread a lesser score which reduced the rating from excellent to average, so effectively it was "downvoted".