On coffeescript's behalf, I've never felt it was a big shortcoming for coffeescript. It really helps to have the actual coffeescript lines, but coffeescript compiles to such a clean javascript, that it is not hard to find the which lines are which.
Emacs mode[1] from Github co-founder Defunkt also makes it really easy to compile a single method/function, so you can easy get the hang of how the compiler works.
But this is great news for production js, which is usually minified, and for other languages, that may not compile down to such a clean js.
I think the big advantage of coffeescript is it gets rid of the whole "implicit global variable" thing. It's not a completely new language (like pyjamas - a python-js converter) just bit of syntactic sugar, which happens to make mistakes a little harder.
Emacs mode[1] from Github co-founder Defunkt also makes it really easy to compile a single method/function, so you can easy get the hang of how the compiler works.
But this is great news for production js, which is usually minified, and for other languages, that may not compile down to such a clean js.
[1] https://github.com/defunkt/coffee-mode