One of the main differences between TeX and WYSIWYG editor is in the way their algorithms functions. Word processors perform optimisations algorithms that try to produce good typography, but at the end of the day, they have to round some of the calculations and give up on doing others because it would take too much machine time, and they need to be able to display large documents instantly.
Some things like proper kerning (space between letters) or character protrusion (punctuation and hyphens advance a bit into the margin) are not performed on a routine basis – same for complex rules such as the one that prevents or limits the number of consecutive hyphenated words. And because the point of a WYSIWYG editor is that the text will print out “as is”, there is no definitive solution to that problem.
On the other hand, TeX has all the time it needs for compilation. It works per paragraphs, trying to find the best arrangement of lines using a system of badness: it assigns a penalties or demerits to lines based on typographic criteria, and it produces the best solution it finds. It produces an overall better result (most word processors are trying to produce the best line possible with what they think the line should be, whereas TeX tries to produce the best paragraph possible while rearranging the lines).
TeX being entirely customisable, you can access each and every penalty or demerit and change their value – this allows you au mostly automate the typographic work. Word processors offer some customisation possibilities (a.k.a. please turn-on hyphenation), but you usually won't get as much precision.
Now, you can get very similar results in most professional publishing softwares like Quark, InDesign, etc. They usually have good algorithms to begin with, and you can (will) tweak every detail by hand. For me, this is one of the big differences… most of the time (La)TeX is going to save you time because when properly set up for your language and page layout, there will be very little optimisation left to perform by hand.
Almost every designer or typographer book that I got my hands on was made with a publishing software and not TeX – the result is either equally good or better. But the authors all spent time looking at each and every line.
In my library, I can tell which books have been printed directly from MS Word or a software that had not been properly set up, and which ones have been printed using a fair publishing software but not much human tweaking. The difference is just huge. Yet none of them equals a good old TeX document (because publishers no longer spend much money on typesetters).
To answer your question: NO. TeX does not have a unique feature. All features were invented by the typesetters of old, who published books of an even better quality than any software. We are merely trying to reproduce these features.
The real question is:
- How well do we reproduce these features;
- How much of the human typesetter do we still need to produce a good result; and
- How much of that human labour are we able to pay for.