I can understand very well how slow and crushing can be heavy. But the way a riff is played, what kind of scale/mode it is written in, and the kind of distortion that is used, are also all important factors. A riff needs to be atonal, dissonant, or in a minor mode to be heavy - this gives it that dark, gloomy, or evil nature that it needs to qualify as heavy. Nothing in the panatonic/blues scale can sound truely heavy, since that scale doesn't sound dark enough. Like the main riff to Metallica's Sad But True (shitty song btw, but a good example that everyone should be familiar with) is very slow and very bluesy, with plenty of distortion, but not at all heavy. I'm starting to ramble now, but I think you get the gist of what I am attempting to articulate here.
