Well, mithril requires a LOT of array literals and object literals in your function calls. http://mithril.js.org/getting-started.html shows todomvc in mithril. That template would be much more compact, uniform, and readable with a lispy syntax.
I've actually considered the idea of hacking a JSX compiler to spit out Mithril output.
If you're going to mix mark-up in your JavaScript, may as well do it with "real" mark-up instead of some imitation language. I'm still on the fence about whether I like mixing mark-up and code, but I'm leaning more toward at this point.
It did seem like an obvious thing to do. I probably would have searched for and found this before doing any actual work on JSX, but I'm glad to know it exists already. :)
