http://www.verticalscope.com has a system that I really like here: http://www.wranglerforum.com/f14/pro-comp-jk-helldorado-wheels-17x9-525905.html
Scroll down to the bottom of the thread and see that they present you the quick reply box, registration form and login form at the point where someone who has read a topic is likely ready to reply to a topic.
xenForo should present the same behavior. Right now you have small, barely noticeable text at the end of a topic saying you must login or sign up to reply.
Why not present them the quick reply form and below that present them tools to register or login to create the reply to the topic.
The Post Thread button can link them to the create topic page where the same options can be given.
Screenshot of how they do it: