I think Snidely nailed it here. They really play, they really sing. There is a refreshing and authentic feeling to their music, which I also happen to enjoy.
What it isn't is the kind of garbage that has passed for music for so long now - digitally altered voice tracks for performers who couldn't carry a tune if it was wrapped up and put in a paper bag for them; pre-recorded music tracks that get lip synced to; the style over substance tripe that so many sheep seem to flock to (ie. good looking performers who dance up a storm on stage with tons of backup dancers and all kinds of flashy stage costumes & props - but have little or no actual musical talent). I'll take a real band like Mumford & Sons, grotty looking as they may be, over the Justins (Beibers/Timberlake), Britney Spears, Katy Perry, etc. any day. They may not be the greatest band ever, but they are way better than a lot of what's popular out there now. Guess I'm spoiled as I grew up in an era where talent and skill counted for something!