The difference in weight of the 2 cameras with kit lenses is 1.4KG for the D600 and 630g for the OMD EM5, thats over twice as heavy. Granted the 50mm 1.8g reduces that difference but then it also reduces the flexibility of the D600 dramatically. The physical size difference is very simliar with the OMD being much smaller that the D600.
As I've said the image quality of the D600 is fantastic, but then the OMD isn't exactly bad, and from the test shots I have is comparable to the likes of the A55 that I've had previously, maybe not quite with the same depth of field, but then thats not exactly the be all and end all of photos. Responsiveness is near instant as is the AF, apart from when using AF-C but then most mirrorless cameras struggle with that and I don't use it very often so not a big worry to me.
The biggest concern with all this is having a decent camera with me when I go out and about with the family, etc. The D600 to me feels like its intrusive, so half the time I don't want to pick it up as its so damn big. With the 70-300 lens mounted its massive and very heavy, particularly to lug about all day.
The smaller systems from the other manufactures suffer from size problems as well, as the body might have shrunk, but the sensor and therefore the lenses are APS-C sized so there is no real advantage to those either.
I need to have a proper play I guess with the 2 and decide if the reduction in quality is something I can live with if it means that I have a camera with me rather than not.