You know, the funny thing is, most metal fans are introverts. They are forced outcasts simply because they don't follow trends (the mainstream). However, when they become musicians, they can get up on stage in front of hundreds/thousands of people and perform.
As a performer, I'd assume you'd need to be an extrovert of sorts, but I don't think it's true. Indeed, some of the funniest people in history have been introverts/depressive in real life (John Belushi, Tony Hancock, Chris Farley, Charlie Chaplin, etc). So metal musicians have the guts to go on stage at Wacken, but behind doors, they're meek.
I bet behind the scenes, Wacken is full of bands with massive stage fright. Everyone vomiting into buckets, changing underwear, trying to remember lyrics etc. But then it gets primeval - strap a guitar around your neck, turn up the amps, hammer out a few chords, and you become a god.