More readers feed the popular thread:

Even the worst player on the worst team at the professional level is so far off the right side of the athletic bell curve that it's as if he's playing a different game entirely than the one played in high school or college. They're freaks – and I use this word in the most positive way possible. This is more pronounced in the NFL, I think, than any of the other major American sports leagues.

Football players are simply universally strong and fast. I mean, look: the heaviest man in the NFL right now is probably Vince Wilfork, who is listed at 340 pounds. (This is a polite fiction; he's probably closer to four bills.) Most people who weight that much are barely mobile, but here is Wilfork intercepting a pass against the Chargers last year. 

This is a mountain of a man moving with lateral quickness, jumping to tip the pass, balancing himself in order to haul in said tip, and then running down the field at a speed you and I will never dream of reaching unless you have a track background I'm not aware of. He ran the 40-yard dash in about 5.21 seconds – that's a almost 400 pounds of person moving at a little less than 16 miles per hour. And he's considered somewhat slow.

You can't just find a guy like that without a massive talent pipeline. Right now, you have thousands of high schools sending athletes to hundreds of colleges, all funneling their best athletes (after four years of pretty high-end training) to 32 football teams. If people start getting scared away, maybe the Vince Wilforks of the world will decide to take up basketball or baseball instead. The quality of play will decline – slowly at first, but eventually it'll become noticeable. Maybe this will take 25 years to become an existential threat to the league, but it's certainly not out of the realm of possibility.

Another writes:

This link is to a TERRIFIC Frontline episode on high school football, and it has a segment on brain injuries. The most frightening part is a team of doctors who set out to do a study of changes in cognitive functions of high school players in the hours and days after a concussive head injury. But there weren't any significant episodes in their first couple days so they decided to put their time to use by doing some baseline studies of cognitive functions of the players before any injury.

One of the things they learned was that there was a measurable decline in cognitive function just from the ordinary helmet-to-helmet impacts on every play, even when there was no concussion suffered by the player. They tracked this group of players for several weeks of practice, and measured the decline in their brain function – thinks like short term memory, pattern recognition, etc.  The concluded that the cumulative effect of all these sub-concussive impacts of the brain on the inside of the skull lead to the same conditions as a concussion.

I've watched it a couple times now, and one decision I've come to is that my very athletically talented 10 year old will never play football.