The difference in
sensor size is the most significant thing. Full frame captures 3.84x more light (area) than 4/3 sensor. And legacy lenses will have the field of view that they were intended to have on the K1. Everything is telephoto pretty much on 4/3, I need a 24mm just to get pretty much a normal field of view on my lumix G1.
Perhaps some K1 owners who use legacy lenses can offer insight into how well the OVF handles visual focus. On my K5 I use a 1.36x VF magnifier by default, but even with that visual focus is often difficult to nail. K-r was the same - I used a split prism VF screen canniibilised from an old ME super for a while on that. Focus confirm signals were often a pic saver.
LCD screen on my K5 magnified is not nearly as clear as the magnified view through the EVF on either my G1 or my samsung NX20.
Two more points: OVF like my the K5 is so much better for spontaneous photography, both my mirrorless cameras have inherent delays that mean the shot is usually missed. EVF is so much better with manual stop down apertures, doesn't go dark like an OVF, autocompensates. And with the strong evf magnification I can often leave a lens like my FD 400mm on f5.6 or even f8 and still see focus.