Naive question: why do the opening acts only get half the screen?
When Knocked Loose and Gojira played, they only used the upper half of the screens, while the lower half showed the London logo (or the logo of whatever city they were playing in). But when Metallica came on, they used the full screens. It felt a bit petty to me.
On the positive side, the sound quality was essentially the same for Gojira and Metallica. It was about what I would expect in a stadium, not particularly good, but not particularly bad either.