I dont get near as tired racing the 450 as I did on the 250 2 stroke. I make less mistakes, and feel much better at the end of a moto racing it than I ever did on the 2 stroke.
I think that is the problem. All the riders are going faster, longer. In the pros, remember when a few of the top guys would bust something out that nobody else would? Now everyone does the same exact jumps and rhythms, even the 250fs. When someone busts out something different it's usually Stewart and it's something really crazy that we are just waiting for him to eat it on.
In the Amateurs, it's even more prevalent. I know many people that are flying off jumps on 450s that really don't have the entire skill level to do so. On a 250 2-stroke, they would never attempt that because they wouldn't feel comfortable, but on the smooth, easy to ride pig they can do it. Then if anything at all goes wrong, they don't have the skill to recover and have a horrendous crash on a hugely overweight motorcycle.
I've been racing for 25 years now, and have had the worst crashes and injuries of my life in the last 8 years or so. That coincides with me switching to a 450 and going faster than I ever did before. Before that the worst injury I had was a broken finger from tangling with another rider on a 125.