I've got a large bookcase mounted on the wall above my bed (not by me). It will probably come loose and kill me one day. At least my obituary will be more interesting than the average.
I don't have any decorations in my guest room. I've thought of adding some, but I've never been a big decorator of places. Maybe some nice prints would improve the look of the place. I think having a print come loose from the wall would startle some guests, but hopefully would not be harmful. Though it really would be better to put it in a frame, and a frame falling on someone could possibly hurt them.
Speaking about mounting...
Sidebar: I expect some of you have wall-mounted modern LED/LCD TVs in guest rooms or in common areas. Aren't you afraid that these may fall on guests? They're a lot heavier than a painting.