Maybe the modern equivalent of "boot straight in to a REPL." is starting a browser, everybody and his grandma knows how to do that, and if that is done, Javascript is closer than any other programming language. I can imagine a default, browser based Javascript REPL just being one mouseclick away. that can be done and probably will be done sooner or later.
Cmd-Opt-J[0]. While easy to get to, it's not the same as being forced into a REPL every time you use your computer; you still have to know it's there, lessening the temptation to jump right in and break things.
[0] or insert browser/platform appropriate shortcut for JS console/debugger here
