The main thing I would like to see in this guide would be "how is DW different from blogging, or from facebook?"

From my experience on LJ, the fundamental difference is that everyone in your circle is both a journaller and a commenter, which differs from blogs where people are either the host/blogger, or a commenter on the site. There's a sense of equality and reciprocality: I share my life/work/writing/creative works with you, you share yours with me, and we comment on each other's stuff as equals.