didn't even know this until I saw a "5 Star Thread" (which just happened to be one of mine)
When you enter a thread, near the top is a drop down arrow labeled "Rate Thread". You can drop it down and vote.
As an example, I gave Matsa's fine thread here
http://www.infekted.org/virus/showthread.php?t=28103
. . . a worthy 1 Star !!