You are right, RR. "Crack the Skye" is better than "The Hunter", but "The Hunter" didn't feel like a let-down to me. I played that one endlessly when it came out and still listen to it regularly. For me personally, the first four Mastodon albums are not far apart as far as quality goes.
Let me just throw in there, that I saw Mastodon perform the whole "Crack the Skye" album live in 2010. Holy shit, that was AWESOME!