Let a thousand coffeescripts bloom. :)
* reduce portability
* add an extra layer of complexity to building and debugging (that abyss is going to be even deeper)
* make code less readable
I agree that there are concerns about debug-ability, but as any Lisper will tell you, you just need the tools to support it. That's why macros need to be native. All the debuggers will be aware of them, so it should be a non-issue.
The whole point of macros is to make code more readable, too. I'm not sure how you can argue that its less readable. Most people agree that the tradeoff is readability/debug-ability.