Mumford and Sons

Simone Joyner, Getty Images

When this band formed in 2007, they bonded over their love of rootsy music. Comprised of four multi-instrumentalists, their first two albums were smash hits in the U.K. and the U.S., garnering exposure for the West London folk scene.

In their favor: Folky bands tend to stay together a while, like Peter, Paul and Mary (now Peter and Paul -- Mary passed away in 2009) and The Kingston Trio (albeit with no original members).
Biggest threat: The "& Sons" contingent gets jealous of Marcus Mumford's prominence and stage a mutiny.
Rating: 25