.EQ
G(z) ~=~ e sup { ln ~ G(z) }
~=~ exp left ( sum from k>=1 { S sub k z sup k } over k right )
~=~ prod from k>=1 e sup { S sub k z sup k /k }
.EN
$$G(z) = e^{\ln G(z)}
= \exp \left( \sum_{k \geq 1} \frac{S_k z^k}{k} \right)
= \prod_{k \geq 1} e^{S_k z^k / k}$$
Though really I'd guess that TeX's math notation caught on as the de-facto standard for "ASCII math" because TeX as a whole caught on, rather than because of its ASCII-math representation being better.
Not necessarily a feature. The eqn syntax does contain more letters and less symbols, but that gives it very low contrast, making it hard to scan the equation and "see" the math symbols through the formatting. When writing a large equation, it helps to have a large contrast between words/symbols and formatting commands. The TeX punctuation helps with grouping and delineation.
In that case, I would argue that it's not easy at all with (La)TeX. The obvious difficulty is people mucking about with cat- and char- codes; but, even if that's not happening, you've got things to worry about like old-style `$ $` and `$$ $$` delimiters. I've never seen a highlighter handle `$\text{$math$}$`, or even `\(\text{\(math\)}\)`, correctly.