I agree very much with what whitecleats posted. I would add nerve to that equation too. I really think the nerve is much thicker in general and this perhaps is why we see more working ability. Conformationally I'll add that I also appreciate that the markings generally tend to be nicer, you don't find too many light eyes or noses. I think the bone is a bit denser, but don't think they are generally larger. I think our american dogs have more ribspring and therefore a larger chest in reality.
I have slowly started to add some American blood into my lines - 25% on one breeding. I have had very good results so far. We shall see what the future brings if I decide to add more.
I don't think heart health is any better, and I think there is maybe more eye disease, although I have not seen it. That may be because they test for it over there like we test for VWD.
This doesn't need to turn into a war, it could be a great discussion for all breeders!
~Lorna