Sometimes dependencies break, url dies, resources 404, and that useful observable notebook doesn’t work anymore, but there is no way to tell the author, or to discuss with other people like you would with a github repository or gist comments.
Maybe you would like to give your opinion about an observable and give praise to the author, and sure you could probably do your own disqus cell in observable itself but come on, that’s ugly, it’d be nice if every notebook had a separate discussion page. And if authors decided they didn’t want it, they could just disallow comments on their books, but I think it should be the default.