One of the things that keeps people from jumping onto DirectAccess is the idea that you need a native IPv6 network in order to make it work. The fact is that this is not true! However, some people think that you need to have ISATAP full deployed on your network to provide a “replacement” for the IPv6 network. In this article there is a discussion of ISATAP and how it fits into your DirectAccess plans
http://www.ivonetworks.com/news/2013/10/is-isatap-required-for-directaccess/
HTH,
Deb