Opeth is a great band and their records are at once increasingly challenging and satisfying. They were never just another black metal band, and it's obvious with their more progressive recent outputs which have discarded the growling vocals altogether.
Unfortunately, a festival is the wrong setting for them. They are not exactly a charismatic bunch, prone to stage heroics or working a crowd. They are best seen in a small or mid-size venue and not in an open air field, in the daylight, where the intricacies of their music is lost on drunken festival punters who only responded during the more brutal, old school death metal passages of their music.
Fans who know their music still enjoyed their set while patiently waiting for headliner Ozzy Osbourne but this band deserves better.