The album is pretty great. I don't know though, I'm still not 100% on the band wagon for Mastodon. I would consider myself a fan, and I like the albums they have released, it's just something that keeps them from clicking fully. If I would guess, I would say it's the vocals. And even then it's hard to describe it. Because it's not so much a problem with the kind of vocals or the vocals being bad, the vocals are just sorta there and doesn't elevate the music IMO. The music itself is great for the most part.
I would probably place this album in my top20 for the year tho.