I had that idea in the past but came to the conclusion that it was redundant because the number of replies/views are already adequate indicators of interest in a thread. I compared the number of subscribed users to the number of replies/views for a group of threads and found that all of those stats scale up at approximately the same rate.
Can we subscribe to a particular individual's post whenever they post? I know you can subscribe to a thread but how about an individual. For example, I'd like to know when stonedinvestor or apex82 write something.