List index: non-public lists invisible to their list owners/moderators

The list should also be visible to anyone who has been subscribed to that list.

Yes and No. I'm not quite sure.

The "Yes" part:

A complete index should contain all lists the current user is allowed to subscribe to or is already a member of. It's obvious, right? Otherwise the index wouldn't be complete. If I know a lists exists and cannot find it on the index page, I will assume something's wrong.

The "No" part:

To some users it might look like an error if they see a private mailing list listed on a page they assume to be publicly accessible. It probably depends on whether a user is aware of the fact that the index page shows private lists *only* when they're logged in (and only to members), but shows *no* private lists when they're logged out.

List owners will potentially hate us for that when they start to receive regular complaints like: "I can see <email address hidden> on the index page! Wasn't that supposed to be a secret?!"

A possible solution: Show private lists to logged-in members, but clearly mark them as private, in a way that makes it very clear they're not visible to everyone.

Also: please note that the list index page is not the only entry point to access list info pages. If private lists are not listed on the index page, members can still access a lists info page from their "My Subscriptions" page.

Re: "please note that the list index page is not the only entry point to access list info pages."

Although the list index page is not the ONLY access point, it is AN access point for SOME lists.
Having an inconsistent access mechanism is counter to accepted Human Interface Guidelines.

Therefore, I suggest that the view provide the "private" attribute as a part of the context that gets rendered.
The template can render this as a part of the css class on the entry and the site's css can customize the display (including hiding the element) however the site designer desires.

A (hopefully accurate) conclusion of a recent IRC discussion (07/14/2013, Terri Oda, Stephen Turnbull, Richard Wackerbarth, Florian Fuchs) on the subject:

*For now*, private lists should be displayed in the list index, if the logged-in user is:

- a superuser
- a member/owner/moderator of that list.

BUT: Private list index items need to be displayed in a way that makes it clear to the logged-in user that the list will not be displayed to non-members.

In the long term Stephen suggested a feature like "named policies" in order to improve the simple public/private distinction. For instance this could add a more strict level of secrecy to the the list options (never shown in list index, shorter session lifetime etc...).