As someone who has been riding motorcycles for about the last 10 years and have done dozens of track days, I'm all to familiar with crashes at the race track(been in one violent one myself). But I'm a huge fan of MotoGP, remembering seeing Valentino for the first time back in 2002(when I really got into bike racing).
This really sucks and I liked watching Marco race. He was aggressive and charismatic like Rossi. What sucks is that Rossi and Simoncelli were great friends and it's possible that it was the impact with Rossi's bike that caused the helmet to come off. The initial impact with Colin Edwards forced Marco down to the pavement, then Rossi with no where to go ended up running right over him. It was so fast and so split second that no one could have avoided it..
So sad and I couldn't believe what I was seeing watching the race the crash was horrific especially the replay I saw later from another angle Marco had no chance. MotoGP has lost one of the rising stars, a charismatic and brilliant rider. RIP Marco Simoncelli.