Not to speak for someone else, but I feel like Mister Tee's reaction to this one might be similar to mine.
Belatedly (I only watched it tonight), I can tell you your instinct was on the money. I'm not opposed to long austere movies on general principle -- I quite liked Winter Sleep when I finally got around to it -- but I watched Leviathan with an ever-increasing feeling of "is this movie ever going to amount to anything?" It had some raw elements of a dramatic story, but it seemed to take pride in skipping over any high points, and making motives/details as opaque as possible. (Just for instance, what was the point of the gunfire heard off-screen at the moment Kolya ran to discover Dmitri and Lilya in flagrante delicto? To fool us into thinking something interesting had happened?) Of course I'd suspected as soon as Dmitri put the screws to the Mayor that there'd be ugly retaliation...but then the Mayor specifically tells all his confidantes he's powerless to do anything, and he underlines, Don't do anything to the lawyer. How does that square with his participating in a public beating? And then the fact that he appears (though the film, reticent as ever, won't quite say it aloud) to have framed Kolya? It just seemed a cheat. Plus, didn't the plan's success rely on the fact that Dmitri was gone from the scene -- something that may not have happened absent the discovery of the affair, something the Mayor couldn't have known would happen?
Ah, the hell with it -- I'm giving the film more of my effort that it deserves (and I agree, dragging in the Book of Job in the last hour to justify the film's content was rather desperate and done half-assed). It amazes me how many people raved about this thing.