I had some time to check on the 4020 series tractors on the JD website. I see 2 problems for MOST of us. First, those are pretty big machines, over 40hp, and MOST of the people on TBN seem to have 40hp or less, and most of those seem to be in the 20 to 30hp range. Second, the loader arms, while curved, SEEM to actually be above the top of the hood, without seeing it in person it is hard to tell if they really are.
As for the question of SIZE and visibility, I do agree, taller people can see over a hood better than shorter people, but at 6' tall, I am taller than the average American male and I am the one doing the complaining about my own tractor!!! So I'd say it is a valid bit of criticism. And again, I don't own a JD tractor with a loader, but I did get to play on some, and I don't think they are any better than the Kubota for visibility.
Loader productivity boils down to a couple of things. First is the ability to manuver, and a Power Trac beats the living daylights out of EVERY brand of tractor on the market for manuverability except for Ventrac and Steiner. Those 3 brands are so much more manuverable than traditional tractors that you can't believe it until you actually get to drive one, and then you are amazed (my brother recently drove my Ventrac and after an HOUR came into the house saying it was "just like driving a video game"
But the Power Trac has the best loader set up of those 3 brands for visibility.
The second biggest factor for loader productivity is visibility, and there only the Power Trac stands at the front of the line by such a wide margin that you have to look backwards a long way to see anyone occupying the #2 position.