with all this talk about cutting and boosting to carve out space (Y) and reverb to add depth (Z), how come no one mentioned panning (X)?
I think that a music mix is 3 dimensional. when you've got three instruments near each other in the horizontal plane (X) like the kick, bass, and lead vocal, there has to be a separation in their major frequencies (Y-axis). That way it appears that the kick hits you in the chest, the bass is in your crotch, and the vocal smacks you in the face. r&b/hip-hop/reggae depend on the ability to deliver that chest thumping, crotch grinding sound while still having that vocalist nail you between the eyes.
the background vocals don't smack you in the face because they are placed further out in both the X-axis (pan) and Z-axis (reverb).
i'm kind of surprised about all of the reverb talk in regards to the lead vocal, because i would think that the lead vocal has the least amount of reverb.
in summary:
when the bass grinds your crotch, the kick is in your chest, the guitar shakes your shoulders, the vocalist spits in your eye, and the pad & cymbals feel like whisp clouds and lightning just above your head... that's good mixin'.
ps. as your equipment (instruments, rig and pre-amp) decrease in quality, the time you're going to have to spend carving frequencies (Y-axis) increases exponentially, because your equipment bled sounds where they weren't intended to be.