This actually wouldn't be too hard...

1. Put a list of ratings on the "newreply" template. 1-10 per se.
2. Store that rating with each new post (would require a new field)
3. Your average thread rating would be the average of the ratings for each reply.

To pull this out just do a query before you display the thread info.

SELECT avg(rating) as rating from post where threadid=$threadid

The rest I leave up to you. Not hard if you know a little PHP.