If there are features that don't work without signing in, then I think that unregistered users shouldn't be able to see them UNTIL they sign in. You can list them in a feature list or something, but don't make them available as links for someone to click on and not see anything in.
For example, if you don't sign into facebook, you can't see a wall or profile or anything, but if you sign in, everything is available.
For example, if you don't sign into facebook, you can't see a wall or profile or anything, but if you sign in, everything is available.