Fango was absolutely correct. We had a problem with people (mostly lurkers I'm guessing) abusing the thread rating system, so we chose to limit who could access this feature.
However, I'm glad you brought this up because I think people who have contributed 2MB or more of attachments can probably be trusted with this great power.
I just enabled the thread rating option for all members who have posted over 2 MB of attachments. Let's see how it goes.
Thanks,