Sonic Highways on HBO is a great documentary following them as they recorded different songs in cities throughout the country. I think when they were in NYC they walk out of the studio and into a father and son walking on the street. Dad tells him they're fans, and the son has been learning guitar, including some Foo Fighters stuff. Dave takes them straight into the studio to show them around. Dropped whatever it was he was going to do to be cool for a dad and his son, that stuck with me. Sonic Highways shows how big a fan Grohl is of so many different musicians, and I think that's key, he's a fan, and he appreciates his fans. He's going to be one of very few musicians in the rock 'n' roll HOF twice for different bands, well deserved.
Another moment from Sonic Highways that stuck with me was when he talked about playing his stuff for Kurt the first time, and Kurt was happy for him. I think he said he kissed him on the forehead. That was from the Seattle episode.