I think it's tough to discern whether or not Mumford and Sons is really, actually popular. Numerically, maybe. A lot of people listen to them, but I rarely find any references to them, nor do I find people talking about their musicianship or newest releases. They're just not that kind of "interesting".
In that regarg, Aziraphale, don't you think it's harsh to call a band derivative? I mean, they're just some dudes who make music. Sure, it's not especially unique or all that interesting, but I'll bet 20 grand that they don't do it for any other reason than having fun making the music they do.
Coining them as derivative is disregarding of the entire process of musical creativity. I mean, sure, it's been done over and over, and I think they know that they're not unique in sound or execution, but they still have fun, I bet. Regardless of people finding their sound to be repetitive and bland (something you can say about most any band in anyone's playlist of favorites), they do what they love, which is make simple-sounding music that happens to have been done a thousand times over.
Not to attack your opinion, but I think a band that still takes to creating music with their real voices and instruments, engaging in a classical process of creativity deserves more than the term "derivative", even if the product is anything other than original or novel.
To relate all I've said back to The Decemberists, I do agree that they are more unique and original in execution, but I still don't find anything they do to be extremely engaging or enticing. There is nothing in their music (at least in the new release, the last one I heard of them was quite a while ago), that has ever strucken me to be especially deserving of recognition. Not to say they don't earn the wage of their work, they certainly do.
Yeah, I'm mostly a fan of The Crane Wife and various of their earlier albums. If you're into folk it's nothing special as you have all mentioned, but to be honest, that isn't the point. It's the lyrical themes that really stand out to me. Colin Meloy has a talent for creating coherent worlds and/or visions of past lives that few can rival, to be honest.
That and he's basically a walking thesaurus.
Perhaps I'm missing the point of the Decemberists. I usually seek for sound in music, in harmony between all instruments, vocals included. Especially when it is something such as indie music, where the lyrics stand at the very bottom of the list of things I pay attention to when listening to. I guess I'll give it another listen, but usually if I seek lyrical musicianship I walk to hip hop and rap in all of its facets and glory.
And I really like Gareth's list. It is especially good to see someone appreciate RTJ's novelty (: