"View all items" temporarily
Google Reader (FMM rest its soul) had a nifty usability feature: when all feed items were read, it displayed "View all items" instead of items themselves (just like BazQux). However, it displayed all items only once, not switched feed to "Show all" mode.
Use case in point: I've accidentally scrolled too far, marking everything as read. Or I want to look up some of the recent items to share the link. Now I click button, which changes feed settings and disappears altogether, so I have to remember to change settings back, and do it in another place.
I'm thinking to implement Reeder-like Unread/Starred/All switch to make one-click switching between "show all"/"show new" modes. And I'm also planning to add recently read section. Which should help too.