I think the absolutely fatal mistake CoffeeScript made was implicit variable declarations, and how that worked with variable shadowing. Once it became clear that it was downright dangerous not to hack explicit variable declarations in using IIFEs, the entire language became a clunky mess.
I have a ton of respect for the language and all of the stuff it cross-pollinated into JS, but it’s an interesting object lesson in how a seemingly tiny design choice can turn out to be disastrous.
Basically, if you want to declare a local variable, you just assign it where you want it declared. It gets scoped automatically to whatever block it’s first assigned in. But if a variable with the same name was already declared earlier (for example in the parent block), then the language doesn’t provide a way to say “this is a new declaration”.
So what can happen is that you have some large block of code where, for example, near the top, you’ve said “x = 1”. Then, maybe a few hundred lines down you have a loop, and inside the loop you say “x = getWidget()”. You think you’re declaring a new variable, but actually you’re reusing a variable from the outer scope. There’s no way to know if you’re accidentally doing this except to search the entire enclosing scope.
It’s even worse in the other direction. You have a small block way down in a function where you’ve said “x = getWidget()” and this is all fine and correct. But you need to add something to the top of the function, and you add “x = 0”. You’ve now retroactively changed the scope of some random variable you weren’t even thinking about.
Edit: Actually my memory’s a little rusty, but I think they actually removed any kind of block scoping entirely by version 1, but everything I said above still applies to nested functions, which you tend to use liberally in CS.
I have a ton of respect for the language and all of the stuff it cross-pollinated into JS, but it’s an interesting object lesson in how a seemingly tiny design choice can turn out to be disastrous.