XML format is a non-starter for me. It adds too much cruft to the text; it requires putting everything inside of open and close tags, including paragraphs.
Thats what I thought too. However, there's a plethora of tools that can generate (and parse!) XML for you, e.g. HAML or Markdown. I write all my TeX docs in Markdown and then convert it to TeX, but stuff like figure placement has to be done in weird comments containing TeX, which'd be easier with RhinoType.
Plus it's already been done to death in XML with XSL:FO, which does the job quite nicely. I don't think it'd be too tricky to rig up an XSL:T for the XML format here to produce FO and run that through Apache FOP.
LaTeX is much nicer to hand edit, and if you're not hand editing you may as well use Word or something.
That's the thing, though: environments are not the norm.
I'm writing a document. I want the default text entry to be that document. There's already a lot of noise in my Latex sources; I want less, not more. (And I recognize that we actually agree on this.)
HAML for content: http://chriseppstein.github.io/blog/2010/02/08/haml-sucks-fo... (its not so bad :P)