- Just giving you a new syntax doesn't help catch bugs - but might help you write cleaner / less bugs (e.g. CoffeeScript).
- Creating a new language abstracts you too far from your runtimes and communities - but might help on-board you easier if its an already familiar flavour (e.g. Dart - closer for Java / C# devs).
JSNext is open to interpretation - not everything proposed for the next version of JS actually makes it to browsers. TypeScript only adds support for proposals once they reach stage 3.