Except for creating an HTML-file and understanding how all that works.
> and the OS becomes (to a degree) irrelevant.
This is pretty much true for any language as long as you don't use platform-specific features (fork).
> Further, javascript is an easy language to prototype in, and, at the basic level, easy enough to teach and write.
"Easy language to prototype in"? Both Python and Ruby are equally "easy", but comes with even more batteries included. They also have a consistent object model, as opposed to JavaScript's prototype-inheritance-is-cool-but-let's-mix-in-constructors-from-classical-OOP. There's pretty much nothing like JavaScript's prototypes in any other languages (Self/Io works quite differently), and I don't really see any gain in teaching that to students early on.
I do have to agree with you about the ugly mix of inheritance models, but for the initial teaching phase, you could largely ignore that before moving on to a different language to teach either true OOP or functional programming.
Using chrome? Hit [ctrl+shift+j|cmd+option+j] and you can write JavaScript without any HTML necessary. Other browsers have similar consoles that need no HTML to write JavaScript.
REPLs are fine to experiment with, but it's important to learn how you would write "real" code. Dumbing down this essential concept is just silly.
All modern browsers has JS console. That's dev environment setup freely for you. And if you use FF, enhancing that env with Firebug is just a few clicks away. WIth the rise of web apps nowadays, this is actually a very good move.
Not passing judgement either way, but I doubt you're ever going to agree with each other on this basis...