One of the features that Google Reader was lacking is undoing previous action. I think that every software should have such option. I've had situations where I'd want to get back in time just for a split second. Marking all articles as read might have you lose some hair. Dragging and dropping a feed to wrong folder can make you wonder "where did that go". I am aware that undoing complicated action might not be a trivial task, but it surely will be satisfying feeling for an end user when he realizes that he doesn't have to spend half an hour fixing what he broke by accident but instead he can just ctrl+z.