Stack exchange is more for a quick questions and answers than having conversations. Thats why it fares better for technical topics than topics like theoretical physics.
It seems obvious to me that something for "physics" questions is too broad to be technical. Even "theoretical physics" sounds too broad. On the other hand, there has been little traction for the high energy physics stack exchange: http://area51.stackexchange.com/proposals/6633/high-energy-p... and http://mathoverflow.net seems to work really well in spite of being really broad.