The option of having the subject field auto-filled is certainly promising. However, the subject field cannot be left blank because the Cnet forums support Tree View, in which only a 'tree' of subjects are displayed along with one the contents of the currently-viewed post. Up until November 2006 that was the only view available, and it's still the preferred view of many.
As to the uniqueness requirement, no member can submit a post with the same subject or body two times in a row. This is designed to deter spammers from simply copying-and-pasting a message repeatedly or using an application to auto-fill the fields with a particular message. Personally, I try to include something specific to the particular post I'm replying to, making it more recognizable to others reading through the thread. (It gets hard to follow a thread that has 50+ replies, all of which use the subject "RE: My Topic.")
It sure would be useful if you could managed to automatically fill the subject field or not require it on a reply to an existing post.
The extremely frustrating part is that seems to be a uniqueness requirement!? In a forum single topic usually covers a single subject why should they need to be unique?