One thing we presently do not have is a good open-source structure editor for SGML/XML documents.
The Conglomerate project aims specifically at producing a good DocBook editor. As of early 2004 it is stilll alpha software.
The MlView project is an XML editor, not specifically DocBook targeted. As of early 2004 it lacks documentation and appears to be in alpha.
LyX is a GUI word processor that uses LaTeX for printing and supports structural editing of LaTeX markup. There is a LaTeX package that generates DocBook, and a how-to document escribing how to write SGML and XML in the LyX GUI.
GeTox, the GNOME XML Editor, aims at nontechnical users. But the software is still (as of August 2001) alpha, more a proof of concept than anything useful, and the project group seems not to be very active; there have been no updates of the website between May 2001 and August 2002 (time of writing).
GNU TeXMacs is a project aimed at producing an editor that is good for technical and mathematical material, including displayed formulas. 1.0 was released in April 2002. The developers plan XML support in the future, but it's not there yet.
ThotBook is a project to put together a GUI editor for DocBook based on the Thot toolkit. It may be moribund; the web page was not updated from November 2001 to August 2002 (time of writing).
Most people still hack the tags by hand using either vi or emacs.