"heavy" to me means a riff, generally quite a slow riff, and yet with so much "energy" that it makes your spine tingle and your head bang at the same time. Just listening to the build up to the riff (there has to be contrast else how do you make it sound heavy?) makes your entire body tense, knowing what's coming, and then the riff kicks in and the tension goes but something else more powerful replaces it
I'm not doing a very good job of explaining this. Anyway, feck it, it's in the ear of the beholder