I think the key factor is the fluidity between Dre's composition and mixing.
If you handed him an instrumental with 15 tracks on it, he wouldn't just wave his wand and make the track come to life.. .
Dude's biggest strength, at least from the mid 90s on, is getting the most out of like 1 single instrument, drums bass and crispy hi hats. Every element of his track occupies a distinct place in the frequency spectrum, so the listener can process the minute details of every instrument.
And so he's probably one of the few producers who caters his style specifically towards sonic perfection... his primary goal is compose his beats in a way that allows for the perfect mix.
People talk about the difference between a producer and a beat maker... in hip hop, I think 99% of musicians in hip hop are beat makers.. they learn all the mixing techniques to make their beats come to life, but they see it as a secondary stage in the process. Dre is a producer first.