What did I find out? That the D600 is a _tool_. What I mean by that is that by the end of my rental period I trusted the D600 as much as I do a hammer or a saw. _It_ takes care of capture the photo... _I_ take care of choosing the photo I want to capture. Yes, even in that short amount of time I came to just trust the D600. The metering is so spot on I didn't have to worry about it. The Auto-ISO is unbelievably good (since you can actually set limits on things like shutter speeds and min/max ISO!). The AF is spot on (although I do lament having to focus and recompose a bit... I got used to not needing to do that on my 7D).
It all adds up to a package where even though the ergonomics are a bit off... it simply doesn't matter because you don't interact with this camera as much as with a Canon... you simply set a bunch of sensible limits and then let the camera do its thing and you come out on the other side with incredible photos.
I fully agree with this assessment - this is exactly how I use my D700. I have 100% confidence in the camera being able to meter and AF in no time at all; this combined with the tremendous ISO performance means I can concentrate on getting the shot without worrying too much about what the camera is doing.
You should definitely think about renting a 14-24 next; I had one for a week and it's absolutely mindblowing. Glad to hear you had a positive Nikon experience!