I look at in the context of the theme of the album. Consider:
Set The World On Fire: you've got Satan railing against heaven and God, and issuing a call to war for his supports. He's angry, he wants to lash out, he's *p!ssed off*. What else could it be but an in-your-face kick-you-in-the-balls heavy track? This is also why the vocals are sung the way they are; there were some complaints that Russell had given up his melodic style and gone all gruff and raspy, but when you look at his vocals on the album as a whole you'll see that they fit the emotion of the story at each point.
Then we have Serpent's Kiss: a greasy, seductive, heavy track with a groove, exactly the seduction that Satan is trying to do to Eve (and finished off later in 'Eve of Seduction' - the verse has a 'happy', bouncy style to it, and you can imagine Satan as a used car salesman trying to sell the forbidden fruit. "Come on, it's easy, trust me, it'll be fun").
Come to Paradise Lost, and you've got Satan again, but this time introspective and wondering if he's chosen the correct path. Look at what I have wrought, he thinks, is it good or bad? And if it is bad, I am resigned to my course and can't turn back. Reflected again in the song and how it is sung, with the chorus being a lament from heaven as they look down on the path that their creation has taken.
And so forth. Instead of a knee-jerk "Argghh they've gone all heavy and lost their prog/neo routes", look at the entirety of the album and you'll actually see all those elements in there *as well* as the heavy stuff
Obviously I hold SX to a higher standard because I know they're capable of writing great music.
