I played football for 10 years - seems like a different lifetime now, though - and I've received my fair share of really hard hits. I was a wide receiver who was laid out a few times while I stretched out to catch a high pass, ribs exposed. I walked away from the game fairly unscathed, except for a partially torn hamstring, a broken right wrist, and bruised ribs during my career.
And, yes, I dished out a few myself, including once in college when I hit a defender and he was rolled off on a stretcher after an ambulance drove onto the field to get him.
So I understand which hits might seem really hard and horrible on a TV screen but don't hurt that much, and others that don't look like they hurt much but do.
On yesterday, I saw a hit that I know was just as painful as it appeared. When people call football a violent sport, this is what they are talking about ... Clowney's incredible hit
It also explains why it will be next to impossible to get these sorts of hits out of the game, because if you do, it changes the nature of the game too much, and because these hits are among the primary reasons people keep coming back for more, concussion problems or not.