Blood Mountain Kings Rule Donington
Mastodon manage to combine aggressive vocal work with guitars that have enough bite to them to keep even those who are not normally a fan of very heavy bands happy. There's something very grown up about Mastodon, due in part to the fact their beginnings go back to 1999 as well as the fact their songs have a precision about them that is missing from many newer bands on the Download lineup.
There are also plenty of mid-long lulls (well, as quiet as a band such as Mastodon can be...) to break songs up and add another dimension to their sound as well as giving the heavier sections a bit more of a kick. Overall it isn't the heaviness of their music which is the first thing noticed, it is the technical ability and the atmosphere that is created from it.
Whilst many hardcore and metal bands will often have songs that drag on for minutes upon end, Mastodon actually have a structure that prevents each track from becoming stale and even for people such as myself whose taste in music leans a little more towards the quieter side of rock, there is something very enjoyable about the band both live and on record.