There is a well-known concept in politics of the 'political horseshoe'. It says that as you get towards the extreme left and right (communism and fascism respectively) of the political spectrum the two ends begin to converge. The two opposing ideologies share more common traits than either would like to admit; they both contain an authoritarian tendency, both are resistant to clean elections, free media and to some extent share anti-Semetic attitudes.
I think this theory can be applied to music too. On the far left is metal, the really heavy, grinding, rhythm-based type, and on the right is dubstep. In terms of the people who listen to them, there really isn't much cross-over; I'm not sure I've ever met anyone who really likes both, and most people who like one tend to dislike the other. At risk of generalising horrifically and unscientifically, I'll leave that there,
However, musically there are many parallels. Dubstep is a rhythm-based genre; the syncopated drums and the classic/fucking irritating dubstep 'wobble' are the overriding features. It may be a Muse remix, but this (I think) is a pretty good example (drop at 1.43). Melody and chord progressions are shunned in favour of bowel-moving bass. It if often extremely heavy, to the point of actively annoying someone with a more mainstream taste caught in the crossfire.
Metal does this too. Few things elicit such an immediate "Christ, turn that shit off" response as hardcore screaming over a drop-C-chug, except perhaps firealarms, wailing babies and Justin Beiber. (On second thoughts, 'wailing babies' has Beiber accounted for). The aforementioned drop-C/D-chug is a fairly good comparison with the also aforementioned dubstep bass-wobble. They underpin the song (song feels inadequate in both cases) right at the low-end, keeping things tight and heavy, and are mainstays in their respective genres. The heavier metal gets, the more it eschews chord progressions and melody, until it is down to its essential elements - aggressive polyrhythms and sparse instrumentation - the same as dubstep. Neither is an easy listen, and both are best listened to live, going a bit mental in a low-ceilinged room.
There are flaws with this theory. Unlike the political spectrum, there isn't really a musical spectrum. At its core I don't think it would be too controversial to say it goes metal > rock > pop > r'n'b > hip hop > dubstep, but that doesn't leave space for jazz and classical, which would probably be closest to metal, which frequently takes inspiration from them. Nevertheless, I think the core argument - that dubstep and extreme metal are innately similar - holds true.
