New Issue should be "available" to unauthenticated users
Unauthenticated users should have a "New Issue" link just like authenticated users do. However, this link should not take them to an issue creation page; it should take them to a page describing why they can't lodge a bug until they sign up. It should also contain a link to the login page and registration page. Of course, the page should be at a different URL from .../issues/new to prevent confusion.
This will encourage feedback from people unfamiliar with Redmine and hopefully do a little to kill any perception that it's difficult to provide feedback.