In these days there are many projects whose aim is to bring new languages to the browser by compiling them to JavaScript. Among the others one can mention ClojureScript, CoffeScript, Dart, haXe, Emscripten, Amber Smalltalk.
I'd like to try a few of these out, but I am not sure what I should be looking for when evaluating these languages to see if they are suitable for production.
How should I evaluate a new browser language, and what are the pitfalls I should be looking for?