I've done a (mostly) death metal fest before and while I enjoyed it (it had a line-up including Suffocation, Avulsed, Vomitory, Jungle Rot, Catastrophic, Entombed, Desaster, Yyrkoon, Waco Jesus, Rompeprop, Cenotaph (Tur) and Dissection amongst others) I realised that I actually prefer multi-genre festivals.
The only real problem with Hellfest is that it's in France