Well, summing up the reactions that were read-/seeable here in Germany:
Nigel Mansell: One mistake and you instantly end up in a wall. Formula 1 made an incredible job by giving the drivers chances to reduce speeds as there are large paved areas. I understand Jody. The smallest mistake can cause a catastrophe. When 34 cars drive around a 1.5mi oval with 300+kph, the track is overcrowded.
Jackie Stewart: It is a big warning. Some accidents have been declared only small incidents. There must be more and harder penalties for dangerous driving.
Derik Warwick: Looking at the quality of the grid, drivers need to work their way through Formula 3 and GP2 to get a superlicense. The overall quality of the IndyCar grid is questionable.
Mark Blundell: It's a Senna moment for the IndyCar community. In the time, where F1 has not suffered any driver's death, there have been 5 in the American OpenWheel racing. The series needs to sit down and analyse cars, tracks and their art of racing and and draws its consequences. On ovals, where most of the cars drive side by side with 300+kph, there is no room left for error.
Michael Schumacher: We all say that our sport has become so incredibly safe noone has to loose his life, but sadly, once again we have to realise, it is just a dream.
Niki Lauda: Dan knew about the risks driving 300+kph in these American pasta pots. It is a sad reminder that motorsport is a sport where you can risk your life.
Christian Menzel (Asian Porsche Carrera Cup Champion, driver of the Haribo Porsche in endurance races): Too many drivers think that motorsport is safe. When I raced on the Nordschleife saturday, everyone thought they would be invincible, although there have been several drivers suffering injuries and the ambulance was on track multiple times. It is a sad news. I have to say, I'm not a fan of this, targeting for the highest speeds and reduce the racing to keeping top speeds throughout a lap. When we tried it in DTM, it had really bad results, so I can immagine what has happened, but I seriously don't want to see the footage, it is too far away from what is right to me.
Reiner Braun (German commentator legend): It was an apocalypse. With speeds much faster. then F1 can do nowadays, speeds of 360kph, you can sit in the world's safest car and get injured.