"Mark all as read" shouldn't mark articles I've just marked as unread
I imported my feeds list from Google Reader some time before it closed and continued to read feeds there. And now I switched to The Old Reader, so now I have a lot of "unread" articles. So I'm opening feed by feed and list articles until I reach seen items. I've marked some of them as unread to read them some time later because I've seen them in GReader but also didn't come to actually read them. And once I've reached items I've read in GReader I clicked Mark all as read because there left 100+ items below. But It also marked as read all items I just explicitly marked as unread. I hoped it will preserve them. It would be very helpful behavior. If I really want to mark ALL as read I could just click this button twice.