I was curious how troff/eqn and TeX syntaxes compared, and didn't find any nice side-by-side examples, so here's a quick translation of one from the troff manual (http://www.troff.org/prog.html#eqn):

    .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

In TeX mathematical notation:

    $$G(z) = e^{\ln G(z)}
           = \exp \left( \sum_{k \geq 1} \frac{S_k z^k}{k} \right)
           = \prod_{k \geq 1} e^{S_k z^k / k}$$

The TeX is a bit more terse, especially in using ^ and _ for super/subscript instead of sup/sub. That can make a big readability difference in things like simple polynomials; a^2 + b^2 = c^2 is just that in TeX, but a lot more wordy in eqn. The troff/eqn looks a bit less like line noise, though, especially in things like "from" in the summation, and >= for \geq.

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.