I have a metal-head friend and we disagree about what makes a song heavy. To me, most metal isn't heavy--it sounds wishy washy and/or whiny. The stuff I find heavy is usually in genres like "alternative" or "hard rock" or sometimes "alt metal".
Here is one of the heaviest songs I can think of (by one of my favorite bands):
https://www.youtube.com/watch?v=BzHZrts808c I think the heaviness drops off after about 0:55, but still pretty heavy.
What I think makes the song stand out as especially heavy:
- It breaks key a lot.
- The vocals are delivered in an aggressive style, but they're sung, not screamed, and the vocalist doesn't sound like a kid.
- There's lots of dynamic change--the drummer moves around his instrument and uses its full spectrum of output, there are ample moments of silence that punctuate really loud sounds, etc.
- Some more stuff I can't think of now but will remember later...
But I wonder if it's also partly in the production? Kurt Cobain allegedly produced that album, though I'm skeptical he's really responsible for its sonic quality. For a better example of how production can make a song sound even heavier, I submit two examples, both produced by living legends (and both written and performed by some of my other favorite bands

).
Terry Date:
https://www.youtube.com/watch?v=uLZBhlTXHuo Something about the way the drums are mixed--especially the kick and snare--just hits me in the chest every time, whether I'm listening on laptop speakers, earbuds, in my car, or on studio monitors.
Butch Vig:
https://www.youtube.com/watch?v=Ugio4fFSLFQ Here again the drums are just brutal, but then so is the rest of the mix. But note that it's still very clean and polished--Butch Vig at his finest.
So, use your producer's ears AND your theory heads and let me know what you think. How do these guys get that incredibly heavy sound?