So laden with metaphor and consequence, Moby Dick is an intense and purposeful drag. There's a lot of real tension mixed in with extensive philosophizing (hemming and hawing), lecturing, and worrying. There are also a lot of beautiful passages and genuine forehead-slapping moments to make you consider larger points like fate and reason. 
 Moby Dick is also a challenge to get through. It was the hardest I have fought not to abandon a work in a couple of years and I think I was rewarded for it. If Ahab is the stand-in for mankind, here, in his struggle with Fate, the experience of reading this novel might be a personal metaphor for the same thing. Or perhaps it mirrors the life of these whalers - rare peaks of tension, panic, and striving to break up the 99% flat gray sea, flat gray gruel, flat gray routine. 
 What separates this from something like, say, 1Q84 as a heavy and challenging novel? I don't know if I should construct an answer for that or just leave it open. I think we all know how Moby Dick ends and we read it anyway - knowing that the book is supposed to mean something we invest in the quest to find out what it is. It's not about the narrative payoff so much as the journey and what we learn on the way. Melville scores points for making me care about a cast of characters. While I complain about how authors treat their creations sometimes, and these characters are poorly treated (no doubt), the suffering of Melville's sailors serves a larger purpose than punching me in the gut for an emotional reaction (and critical kudos). 
 I'll be thinking on this for longer than I ought, most likely. Perhaps I'll come back and edit for clarity or changing opinions in a week.