I think it's also the degree of flexibility. Typesetting is itself a creative process as much as a technical one. When I do technical writing with LaTeX (and I do a lot) I'm usually ok with about 99% of what it outputs, but then I have the ability to fiddle around and make that one table have the borders exactly how I like them and that picture caption to be spaced just a little differently etc; I want to indulge my sense of aesthetics just a bit.

It's the combination of (mostly works by default) and (I can make it look just right) that is appealing. It gives me just enough stuff to play with to be satisfying and even a bit fun, without having to tweak so many things that it gets frustrating.