[N.B.: I use a Mac, but I'm assuming the following is not Mac-specific.]
I've noticed that, font-wise, SE sites can be divided into two categories:
1) Those that employ a sans-serif font (e.g., TeX SE, superuser SE, and this one). These display numerals as "lining figures" (~full-height numbers designed to line up with upper case text).
2) Those that employ a serif font. These display numerals as "text figures" (~half-height numbers designed to line up with lower case text) (see: https://en.wikipedia.org/wiki/Text_figures). Here's an example from Chemistry SE:
Here's the problem: Sites like Chemistry SE and Physics SE have many math-heavy posts. The half-height numbers make posting more cumbersome, since they somewhat necessitate the use of MathJax every time a numeral is written, even if it is within a block of text.
I.e., if the poster doesn't use MathJax when writing numerals within text (as in the example above, "0 K", which means zero kelvin), the appearance of the numerals will not only be notably inconsistent with those displayed within MathJax (where they are full-height), but also sometimes confusing to read: For example, on MathJax-enabled sites, "0 K" looks like "o K", i.e., like the "K" is preceeded by a lower-case "o". This is not an issue on sites that use lining numbers, such as this site, where "0 K" looks perfectly fine.
Indeed, within text, I would argue that "0 K" on a sans-serifed site (like this) is not only less cumbersome to type than "$0$ K"——it also looks better: On a MathJax-enabled serifed site, the zero isn't properly positioned (it's too high, and too close to the preceeding text; see screenshot above). It is possible to correct the horizontal spacing, but that makes things even more cumbersome if you have to do that every time.
Given this, why not use a font that displays lining numbers on sites that have math-heavy posts?
And if the standard serif font(s) SE uses are limited to text figures, might not the standard sans-serif font(s) be more suitable for math-heavy sites?
ADDENDUM: What motivates this is the idea that we're all volunteers, and the site should be designed to encourage posting with the minimum "friction". Consider, for instance, this comment (which I added to my answer to this question: Infinite Increase in Entropy when Energy added to Absolute Zero):
Mine is the second comment, made in respose to the question by the OP. As it was a comment, I didn't really want to bother with using MathJax, so I didn't; my focus was instead on doing the work of giving the OP the clarification s/he needed to understand my answer, and getting it out promptly (note the date stamps). Nevertheless, because Chem SE uses text numbers, I had to add dollar signs around every one of the six zeros. When I'm focusing on that, I'm not focusing on the content. That's added friction. And every added bit of friction can make the difference between someone bothering to give additional help vs. not.
With the sans-serif font style used on this site, that comment would have been perfectly clear and readable without the need for the added dollar signs.