Learning how to really listen and understand.
Not much else really matters.
You can emulate things all day long and copy or mimick what other people are doing by using certain pieces of gear or signal flow but it won't lead you anywhere until you can really hear what it's doing to the music.
Once your ear is really trained things like picking the right reverb or EQ settings, having a mono compatible mix and all that jazz...it's a non-issue. Like proxy said...just mix from the gut.