The bottom line is while all the debate about how to improve Javascript is going on, all the while with no real changes coming to all browsers (I'm looking at you, IE), Jeremy went ahead and shipped something that's great that you can use now. As usual, the people who ship will win. Good luck with all the wrangling of the big vendors to change a tiny bit of syntax, I'll be happily sitting here writing CoffeeScript today.
All that's missing is a layer of tooling in modern browsers (Chrome, FF) that make it so you can look at CoffeeScript as if it were in the browser, and the abstraction would be pretty complete. You'll still have to precompile the stuff on your server, which sucks, but practically speaking we're not talking about anything that can't be clean and easy to use without appropriate tools. Generally speaking client side code is short and sweet, so running your system in development mode where the script compiles on the client and doing a full compile server side for production mode isn't that far off from doing a debug vs release build in the world of native apps.
Brendan opened a Firefox ticket this morning to make line-number mapping possible, and that's really the biggest outstanding issue for debugging, since the semantics are otherwise one-to-one. In fact, it would make <script type="text/coffeescript"> tags a lot more fun to work with than they already are, since debugging through an "eval" would be far less painful if exceptions pointed back to the proper position in the original source.
We could go one step farther than line number mapping and just define and implement a standard for full-on support for original source pointers. Then we could implement any number of langs using minified Javascript as an intermediate language.
The bug I filed based on Jeremy's suggestion is about source coordinates, not line numbers. We are way past the ancient mistake of numbering lines but not columns, which bites minifiers all the time.
All that's missing is a layer of tooling in modern browsers (Chrome, FF) that make it so you can look at CoffeeScript as if it were in the browser, and the abstraction would be pretty complete. You'll still have to precompile the stuff on your server, which sucks, but practically speaking we're not talking about anything that can't be clean and easy to use without appropriate tools. Generally speaking client side code is short and sweet, so running your system in development mode where the script compiles on the client and doing a full compile server side for production mode isn't that far off from doing a debug vs release build in the world of native apps.