I think the ergonomics on my little eb1150 is actually pretty good, it could be lighter but hey it was designed a few years back.
I think that ebook designers are making a mistake that some cell phone manufactures made, making things too small. There is a point at which being small is a hinderance.
It seems like many of the turn page buttons are a bit on the small size, which for someone like me, who has little hands, could be a problem, it could also be a problem for someone with bigger hands too. I find that reading on my iphone is a bit annoying because of how I have to turn the page (swipe finger across the screen), especially since I have to turn the page more frequently.
I think that the ereaders are still in their toddler years and that companies have not found the perfect combo of ergonomics yet.
I would say think of keyboards and mice, they have to have a large variety of options because people have different hands and some keyboards work for some people and not for others.