I know we take the process for granted here, since so many of us do it on a regular basis but.... seriously, mixing to a world class standard is freakin HAAAARD. Not only do you rely on stellar performances, recorded tones and possibly even some tight editing, but you also need to be 100% tuned in the whole way through the mix. A 0.5dB bump at 150Hz on the bass guitar can make or flush your overall record sound. An extra half dB comp on the snare can be the difference between punching you in the face and getting lost in the back.
Not only this, but the way the levels and tracks interact with each other is amazing. It's like a microcosmic society inhabited by frequencies that you're trying to micromanage. You need to be on top of everything from the punch of the drums, to the EQ curve, decay of the rooms, room samples, various verbs, delays etc. How the guitars fill out the mid-range of the record, where to cut them for the bass growl to come through, where to cut them for the vocals to sound clear and crisp, how much high mids to add to the vox in order for them to sound crisp and in your face. How much vocal comp is the difference between constantly being up-front, and constantly being drowned out and lifeless. Kick/bass interaction (arguably THE thing that sorts out the men from the boys) above all is one that bends me over and takes it home almost each and every mix I do.
Seriously, this job is only for the masochists among us... No wonder our little corner of the industry is so niche.
Btw, I'll have an alternative/industrial rock thing to show you guys soon