Currently for instance PUT calls to /.rest/workspace/path return 400 Bad Request for all of the following error conditions:

• No name specified for node to create
• No type specified for node to create
• Parent path does not exist
• The request doesn't contain any nodes to create
• The path to create already exists

However, a client can't differentiate between these conditions and display a meaningful error message, because all that is returned is a 400 Bad Request. I'd like to suggest returning an error message when an error is sent, which will make it easier to troubleshoot calls that return an error status code and also will allow client builders to provide a better experience by being able to show a specific error message to the user when something doesn't work.