Well, they normally book larger old bands like Saxon, Judas Priest, Guns 'n Roses, Ozzy Osbourne, etc. That are cool names, but those acts are so disappointing live, most of those bandsmembers are 50 years old and they just cant do a proper banging headlining show. Plus, after 5 years you can repeat your list again ...^Exactly. Headliners suck major dick IMHO. But some really nice bands: Bodom, LOG, Exodus, Legion Of The Damned, Dream Theater, Blind Guardian... Not that bad, but headliners are shit, specially the last day.
That's why i'm glad that they didnt book 3 old 80's bands.
I'm not a big fan of slipknot but they have some good songs and i'm sure they will play a hell of a show, better then Aerosmith 2 years ago.
But:
1. I must agree: Manson sucks
2. Iron Maiden STILL fucking kicks ass live after almost 30 years. But they continuely headlined graspop for the last 5 years.
Possible headliners for the future: Rammstein, System of a Down, etc ...