Is TeX still a big deal? I used it (and LaTeX) for all of my school work 25 years ago but I don't think I've touched it since then. Even back then, most people were using word processing software to create their documents.
Can a variant of an already somewhat fringe piece of software really become a killer app?
TeX is certainly still a big deal, and is not at all fringe. That's my experience from academic Physics and Computer Science. I've heard that different disciplines vary in this regard (e.g. humanities may use more word processing, possibly because they don't need TeX to typeset math).
Anecdotally: when giving a presentation last week, the event organiser was surprised that my slides were HTML (from pandoc; I'm an ex-Web dev), as she'd never seen such a thing. Everyone else used beamer (TeX slideshow package).
One of the most enjoyable moments this past (academic) year was noticing my son's 7th grade homework sheets looked awfully familiar from a typesetting and font perspective. Sure enough, after inquiring, I confirmed that the teacher was using LaTeX to prepare all her schoolwork.
That may be true, but the world of academic Physics and Computer Science people is still a relatively small one.
Anecdotally: in the 20+ years I've been out of school I've sat through hundreds of presentations. I don't think any of them were created with TeX (unless TeX is the underlying technology of PowerPoint, Keynote, Prezi, Impress, etc...).
TeX used to have numerous unique features: fairly good fonts; automatic hyphenation, which makes it feasible to lay out text with full justification; support for non-English Latin characters, etc. The rest of the world has caught up with these.
IMO the ways in which it is still special are: (1) good math typography; (2) integration with automated bibliography systems; (3) easily generatable by other tools. (Lots of things that eventually turn into HTML hit #3, but not lots of things that eventually turn into really good-looking PDFs do.)
These points certainly aren't important in all cases ... but for what it's worth, arxiv.org hosts more than a million scientific papers, the vast majority of which are written in TeX.
It's quite important in mathematics and reasonably important in physics. Almost all of my fellow students in my undergraduate physics labs use LaTeX to typeset their reports, and a quick browse of arxiv should show that it's dominant amongst academics. It's one of the few piece of software that actually typesets mathematics correctly and well. Don't get me wrong, from what I've seen recent versions of Word have improved hugely and even innovated with OpenType math-extensions which have then been adopted by TeX, but TeX still leads the way.
One of the most significant advantages it has over other systems is sensible defaults. Everything from the choice of default fonts down to the margin sizes are chosen to look well enough from the beginning.
