A band who I feel never fulfilled their potential is Trivium. As a 15-year old they were one of the first Metal bands I listened to. Their second album 'Ascendancy' made a big impression on Metal, whether you loved it or hated it, and they were supposed to be the next big thing in Metal. They opened the main stage at Download in 2005 with what is probably one of the legendary Donnington Park performances and from then on they should have gone on to bigger and better things. A year later the vocal style had changed and they had their collective head up Metallica's collective asshole. They were underwhelming when they supported Iron Maiden in 2006 and not much better on their own UK tour a few months later. Gojira were actually one of the support bands on that tour and I was a lot more interested in them. Trivium's third album, 'The Crusade', was a massive disappointment and an attempt to be the new Metallica. It backfired on them quite badly and I don't think they play anything from it anymore.
I have a feeling they were badly advised by people at Roadrunner Records at the time. The label wanted them to be the next big thing in Metal and probably encouraged them to big up Metallica in every interview they gave when a couple of years earlier they'd cited In Flames as being a major influence. Trivium could have gone on to headline Download festival if they'd been better advised, but instead they just stayed as a middle-of-the-road modern Metal band who no longer make an big impact on the genre. It's a shame really. I guess they're probably making the sort of music they want to make now, not that I've taken any interest in them for ten years, and a bunch of 20-year olds who are suddenly thrust into the limelight probably aren't going to make the best career decisions.