FrameMaker has a decent doc compare feature, which may be sufficient
for most people's needs. This is actually one of those things that
could be done externally; use diff & pipe it through a filter creates
a marked-up comparison document.
I'm also fiddling with the idea of implementing a FrameMaker-style
book document for AbiWord using a TCL/Tk script and beefing up the
command-line options (to allow printing the files in a book, for
example). I have several writing projects to tackle before I start
in on coding though.
Larry