Not sure if what you mean by "rating" will be same as me, but here's an idea :
a type of rating we've had to do in discussion boards is for user acceptance testing (UAT) discussions
what I did was create a new board for the user's UAT responses, allowing all to add and view but only change and delete their own
then I added a new field to the discussion board - it may not be obvious at first, I was surprised how many people didn't realize you can do this, but really the board is a list mainly like any other
so I added a field for "pass/fail", each time a user posts a reply to a thread they get their own unique entry to the discussion and each one has its own "pass/fail" field
if you create a new View for your site you can display this custom "rating" field, and even Group on the field, so for example we group by Subject (this keeps all posts within the board together, bcz each threaded discussion has a unique subject, for this case the Subject is the item we are UATing), then by pass/fail, so if you collapse the thread and then click open one level, you see how many passes and how many fails and how many posts total
hope this helps!