Is MathJax staying as-is?
Relevant, yet not the same. Given all those major changes that are coming, perhaps it's time to leave MathJax behind and move to a newer and faster service (e.g. Katex), or is it too much work to do for too little gain? Swapping services might indeed be a bit of an overhead, but it might be worth it in the long run.
The Jobs and Teams sections won't be present on non-SO sites, right?
I honestly hope that Jobs and Teams won't be there on other SE sites. If that's the case though, what is that big navigation bar going to be populated with, apart from Users, Tags and Home?
General remarks about the impact on user experience
As other answers to this thread noted, standardizing everything impacts user experience dramatically, albeit improving the loading time of the page infinitesimally. I don't think it's worth it sacrificing so many nice, small things that make the overall site more friendly to the user. While I do agree with standardizing things like navigation, giving up on vote buttons, fonts and badges is just too much.
Apart from this, the leading navigation bar should really be collapsable, given that it's not really relevant for everyone and it really takes a lot of space.