JS types as comments
I think types are a great feature to begin with—they help to obliterate a whole class of runtime bugs and speed up development with IDE hints. All well & good. But I think that part of the elegance of this proposal is that you can put anything in those new 'comment' spots. It doesn't have to be TypeScript, or Flow—it can be whatever you want. Because the compiler will treat it as a comment, it doesn't need to adhere to any existing syntax.
But so anyway, I think this an excellent step towards closing that gulf, removing at least some of the need for transpilation steps (especially in the age of Interop 2022), and making things easier and faster for folks who work on the web.