Well,
Beyond the "level setting" stuff as covered in that very good sound on sound article, proper gain structure is one of the [million] variables on your way to really great sounding mixes, instead of just okay mixes.
These comments are specific to mixing on a console.
I would suggest REALLY listening to the tone of something like a kick drum, as you turn it up in relation to the bass guitar. REALLY listen. Listen for what pokes out first, rather than listening for an overall gain change.... Now listen to the kick with a compressor on it. ANY compressor. Set the compressor super "wrong" in your opinion. Turn the kick up and down in relation to the bass...Lighten up on the comp a little.... repeat. REALLY, really, really listen to what happens. DONT eq (yet, if at all). I am always amazed by compression, and its ability to let me turn the kick up, because I want more beater attack, but I dont want EQ-y sounding metal beater in this case... SO: set the attack time of the comp you are using on the kik late enough that it misses the beater all together. Completely misses the party happening at the leading edge of the transient, the reacts VERY strongly to the rest of the wave. Amazing. All of a sudden it sounds like you EQ'd on a ton of beater! That is part of gain structure. Get the thwack/boom ratio level'd out to your liking, then bring it up against the bass guitar....
I could go on like this for a long time, but I am tired... I will post more about this if anyone wants to read my ramblings....
Gain structure isnt just important: it is everything you do. It is EVERY SINGLE amplification of attenuation you happen to be involved in, which if you are the engineer: is all of them!