An automatic rating system based on the number of replies would assume more posts means better quality, which is not necessarily the case. A Google type ranking might work ok for informative type topics (though there are already sticky topic listings in place for those), but discussion topics don't get linked/referred to as much. There may not be enough links in the forum for best results, even if Larian wanted to divert resources for that.
I rarely notice the topic rank, so it doesn't matter much to me if it gets used or not.
If the reply button is greyed out in an old topic, near the top of the page hit the buttons to switch between Flat and Threaded topic views, and then back again, and you should be able to reply.
Depending on the topic, it might make more sense to start a new topic and provide a summary and/or link to the old topic. For example, it might be confusing to bring up a topic discussing a bug that has been fixed or situation that was changed in the patch.
