I think it’s the drums. More specifically, the emphasis placed on the drums. In hip hop, for example, the drums are a serious part of the music. If you took the drums out, there would barely be anything left. It ties everything together and is an integral part of the music at the same time.
Rock, in my experience, uses drums to keep time and pop out a sweet solo from time to time. Since the focus is on guitars, you could take the drums out of some songs and still keep a lot of what makes them epic. Take the baseline out of a hip hop song and you’ve barely got anything left.
These are, of course, generalities. There are exceptions to every rule, YMMV, blah blah blah. At the end of the day, though, I think the drums are what do it.
