It's a tough, violent game that is going to take a lot from a guy. People that played in college that I know have lingering stuff. I have lingering stuff that bugs me from time to time at only 24. It's not just from the impact; it's from the years of dedicated weight training as well. Most college players (at any level) who play between the hashes can squat 500 for pretty high reps and hangclean 250+ for reps. The kind of training required to get to that level is pretty intense and leaves it's own mark as tendons do not recover as quickly as muscles. I can only imagine the level of training and conditioning required for professional football players.
When you take the impact into account the toll on the body is pretty high. The level of technology in today's padding allows for some incredibly violent impacts with minimal dings and dents. When something goes in football now it is usually pretty significant. Looking past the body issues, the head trauma associated with the rapid deceleration of hits is incredible. Concussions suck and until recently it wasn't uncommon for athletes to lie to the trainers about their symptoms to get back on the field (the cat, rat, hat sequence for example is easily memorized. Even if you don't know who you are talking to you usually can put together "cat, rat, hat...Yeah, I'm good, just got my bell rung"). The brain issues are my main concern for these guys as it's only going to get progressively worse as these guys continue to get stronger and faster. You can condition bone and tissue to build more armor, but you can't train to prevent your brain from bouncing off of your skull.
With that considered, I don't exactly feel bad for these guys as they are highly compensated. I do worry about them after football, and they are clearly going to have long term health issues. Linemen in particular are going to have dramatically shortened lifespans. I think that anyone with any degree of compassion feels for these guys but ultimately I understand that it is a tradeoff. It is a tremendous career opportunity and typically is their passion as well. With the exception of the brain trauma, I think that the lingering injuries are worth the compensation.