The issues on the github repos for the RAML parsers, the API designer, and other projects are a great place to file bugs, enhancement requests, etc. -- as usual for code projects. On the RAML spec itself, you can use issues to file bugs on the spec itself, discuss enhancements or seek clarifications, etc.
In the RAML forum, you can also discuss concerns about RAML, suggestions for improvements, directions for the language... and express interest in other tools, discuss best practices, find out who's using RAML and how, compare to the alternatives, etc.
In short, feel free to use these forum for any relevant discussions. How do you want to use the forum vs github issues?