To hell with harmonic mixing! The world's greatest DJ's started on pure vinyl and even though most of them use digital mediums now, imagine the groundbreaking innovation that would be lost if it weren't for complete and total experimentation and rule breaking, e.g., using a turntable for mixing or the art of turntablism. Sure, knowing keys can help with finding samples and matching loops and what not, but it should never ever be a rule of thumb. I do mix according to BPM since having gone digital, but I always kept my vinyl in order of the feel of the tempo. That just helps keep the genres somewhat close together. Great post, keep it coming!