Mohs Scale of Rock and Metal Hardness on TV Tropes
This is a great idea. The scale goes from 1 to 11, of course. Go to the page for more detail, but here's a quick overview:
1. Acoustic rock
2. Slightly lighter rock
3. Average rock
4. Slightly harder rock, lighter punk
5. Hard rock, punk
6. Heavier hard rock, lighter metal
7. Classic metal, power metal, hardcore punk
8. Thrash metal, heavier classic/power metal, metalcore
9. Lighter death/black metal, heavier thrash metal
10. Most death/black metal
11. Extreme death/black metal, grindcore
The page goes into more detail on what makes some songs heavier than others. Distortion, speed, instrumental solos, vocal styles, etc. I don't agree with where they put a lot of songs. Metal fans want to put metal ballads at 1 when they should be 3 or 4. On the other hand, the Beach Boys page claims they get up to 5. And there is no way Muse's "New Born" is heavier than "Stockholm Syndrome". You'll disagree with some of the others. The web site can't agree with itself on some songs. The Mohs Scale main page has Metallica's "Battery" at 8, but it's a 9 on the Metallica page.
They say it's hard to place rock bands who don't use guitars. I don't know where Kraftwerk or Depeche Mode would go, but they're barely rock to begin with. Ben Folds Five and Van der Graaf Generator are two rock bands without guitars and they both get to at least 4. Of course they distort other instruments. Ben Folds Five had distorted bass and VDGG had distorted organ.
I say drumming has a lot to do with it. Songs with loud drums will score higher than bands with quiet drums, drum machines, or no drums. There are exceptions. Some extreme metal bands use drum machines.
It's a useful way to think about rock music. A final warning: TV Tropes will suck you in for hours.