I was very surprised to find that *nothing* in the viewfinder is ever in focus in the D800 unless a battery is inserted. What causes that? And are there any other SLR cameras that have this "feature"?

Page 33 of the D800 manual says: "When the battery is totally exhausted or no battery is inserted, the display in the viewfinder will dim. This is normal and does not indicate a malfunction. The viewfinder display will return to normal when a fully-charged battery is inserted."

But I couldn't find any reference to lack of focus ability. I find it interesting that they would remove the ability to use the viewfinder and manual focus without the full Li-Ion battery, but personally I don't mind it at all. (In fact, I'd prefer a completely mirrorless model with an EVF and Nikon-One-style PD autofocus.)