As we've started discussing, Stack Exchange is planning to redo the editor for all questions and answers across all their sites. As articulated by Emilio Pisanty, these changes will have significant implications for MathJax.
In the initial discussion, I thought the possibility that these implications might even include not fully supporting MathJax seemed like a worst-case scenario, which we would only even mention to confirm that everyone agreed it was not a real option. But it now seems that among the options Stack Exchange has suggested so far, the most reasonable one would entail exactly that, as it would involve switching to some different Latex-like framework.
If we go down that road, then we're going to have to get even more involved with things: we will need to think about which Latex / MathJax features we need to have supported on the site. Because if you leave it to the developers to decide which Latex features are worth supporting, then you end up with something like the Microsoft Equation Editor, a sort of caricature of what it means to support mathematical typesetting.
Though this would be a large task, it also presents an opportunity. If we were to switch away from MathJax, whatever new solution replaces it might be an improvement in certain respects.
So perhaps I'm jumping the gun, but I think it may be worth getting started on this task. We're still in the early stages, so there's no way we will come to a definitive or comprehensive answer right now, but we may start to get some ideas. Up to now, I've never had to dialog with a non-mathematician about what I need in Latex, so I'm not really sure what their preconceptions are about what we need. Some things I might guess they don't anticipate include user-defined macros and commutative diagrams, but there are probably much more basic things they'll need to have explained. On the flip side, my impressions of what they will find difficult or easy to implement are just as ill-informed as their impressions of what we will need. So the question is:
Question: Which Latex features are important to have supported on this site? Which ones, if not strictly necessary, would be very nice to have supported?
It's worth considering both features which are currently supported by MathJax, and ones which are not.