Maybe the JOIN DISCUSSION button name should be NEW DISCUSSION or NEW MESSAGE. The breadcrumbs trail actually ends with "New Message". See attachment.
Thanks for catching this along with @David_Burnham. We talked about this today and we're going to go with "Start a Discussion". It should change sometime tomorrow. Let us know what you think.
I've also changed the button on the message page so that it pulls the system string and will be consistent with the button on the top-level discussions page. This should go live at our next push to production; sometime later today.