Since I'm running a bit out of comment space I'll use an answer.
There's tons of questions on Meta.SE about MathJax. The problem if you want to call it is that it's quite dependency heavy and essentially slows down the page to a certain degree. An analysis shows SO losing a third of a second on every page load—and that includes page loads of questions that don't even have formulas in them. Furthermore, that markup would load from the CF CDN which means it might not work for those who have firewalls blocking that.
Therefore I'm not convinced that blindly enabling MathJax is the way to go. Even without any further configuration Stack Exchange offers the full Unicode set of characters and with html-sub and html-sup quite a bit can be reached.
Personally, I haven't noticed enough questions or answers that would profit from a proper MathJax support at this time. If someone can name a handful of questions or answers that would profit from that however it should be considered.
For now I think we'd profit more from syntax highlighting.