I have a cool idea for a multi-day event here on BBG! I've noticed you like when a theme carries over many days and I have fun writing that way, too. But, before I can write about what I want, I need a clear understanding of what "heavy" really is...keeping in mind that all art is subjective.
I think a pounding rhythm section (with overt bass) tends to make a song heavy. A band doesn't have to be "metal" to create a "heavy" song. In fact, there are random pop songs that include heavy sections. I think the lyrics need to be serious or dark. A singer that has a very powerful, booming voice or soaring scream helps. Muted chords on the guitar don't hurt, either. Divebombs are good. Downtuning is good. Distortion helps. Perhaps even the length and general complexity of a song adds to a heavy vibe.
What do you think? What - to you - makes a song "heavy?" Your responses will help me write this website next week.