I've always had an interest in markup and document formatting languages, and have long used troff and LaTeX for document formatting and XML and DocBook for markup, using the DSSSL DocBook Stylesheets and OpenJade along with JadeTeX to produce HTML and PDF output. I've also used a homebrewed application that produced Postscript output from Simplified DocBook using Groff.
Unfortunately, I've not been particularly satisfied with the results. Many of the tools involved are complicated to set up, some are suffering from a lack of manpower for maintenance, and often it is very difficult to customize the appearance of the generate output. Groff is perhaps the simpliest to install, but I've no desire to write in bare troff any more than I have in bare LaTeX, and groff and its macro packages lack good support for PDF - in particular, pdfmarks. LaTeX has good support for PDF (especially when you are using pdfelatex), but it is difficult to customize the appearance of the output, and there are a number of quirks when it comes to font handling.  And the PDF output from the DSSSL DocBook Stylesheets  can be very ugly when you are doing complicated things, and since it uses JadeTeX it's even harder to customize the appearance of the output than LaTeX .
While I've not found any general solution for the problem, I've have run across something that looks promising: ConTeXt. It is a TeX macro package that provides about the same level of structural markup as LaTeX but appears to be much more customizable.  It also has very good support for interactive documents, reasonably good manuals, and appears to have significant development resources behind it. I think it would make a more suitable backend than LaTeX. 
As for those documents where XML markup is too much of a pain (say, things that should be easy to read as plain text, where the verbosity of XML tags overwhelms the text), I've found that the reStructuredText (part of the Docutils project) fits the bill nicely. Every since Steffan O'Sullivan produced the Fudge RPG in plain text format I've been looking for an easy way to turn something that looks like plain text into reasonably nicely formatted output. reStructuredText fits that bill nicely. It has enough structure to be useful without overloading the plain text with markup, it produces good looking output, and it's reasonably easy to process, with a DTD, an XML form, and an extensible implementation that's not too difficult to program. For things that I would otherwise be writing in plain text I'll be using reStructuredText from here on out, and I'm writing an reStructuredText to ConTeXt writer as I have time.
WYSIWYG? Well, that's a nice idea, but I've yet to find a system that works well for anything but the simpliest documents. I'd like to find a portable, open source WYSIWYG XML editor that supports DocBook, but they seem in short supply.