I'm a bit wary of the proposition, as I think mixing is kind of a game of inches, and no one particular thing provides a huge difference on its own.
 
However, I'll throw in this: once I decided never to spend more than a day on a mix, the rate at which I got better went through the roof. Coming back and making refinements on a recall is fine. But if I'm generally not happy with a mix, I never tweak what I've got, I flatten it and start again.

Building effective sound absorption, ARC, thus reference material and my mixes more accurately. A very close #2 is CUT - DON'T BOOST! There's a large number of small things that contribute that is growing steadily in my world.

Learning how to gain stage properly
Cutting instead of boosting
Room treatment (partial)
Listening to mixes on as many systems as possible
Not being afraid  nuke entire clips

konradh
1. Use a High Pass Filter on almost every track to clean up the sound but also to free up headroom.  Example: Acoustic guitar thumps and rumbles below 100-150 HZ just take up space.
2. Make sure the bass and kick complement each other.  Example 1: You might cut the kick around 300 hz to eliminate muddiness, but either boost the bass there or just leave it alone so it is defined in that range. (All depends on the song and instruments.)  Example 2: Decide which of those sounds is going to represent your low-end power and get the other one out of the way.  You don't want a lot of 40-80 hz on both sounds.
 
You can see from this that I consider getting the low end right the most problematic area. 

I recently stopped high passing the crap out of everything and I think it sounds better. What I do instead is give things a moderate HPF cut around 80Hz or so (higher depending on the frequency range of the instrument), and reduce the low frequencies where necessary with a low shelf. I think if you HPF too much you lose a lot of the warmth in the mix and things end up sounding too brittle. Whereas if you use a shelf to attenuate the lows instead of getting rid of them completely, you stop them muddying up the mix whilst retaining some of the warmth. 

I've learned SO SO much from this group over the years. I think one of the single biggest jumps in my mixes came from learning how to gain stage - in particular how to use VU style meters to gain stage. There is a great thread that really made a difference for me...

I know you asked for (at least implied) the _one_ most significant gadget/event/feature, but I only have the energy/initiative to narrow down to four.
 
1) The release of Cakewalk Pro Audio, meaning that I (and countless others) could go non-linear & non-analog, ditching the tape recorder and the smtpe striping and the splicing block. For better or worse? You decide!
 
2) The "Project 5" era, when Cakewalk tried to expand outside its box (and its comfort zone) into new audio technology areas. With the actual P5 program, I include all the RGC audio stuff that came with it, including those trusty synths that became, and are still, a mainstay of creativity for me .
 
3) The regular inclusion of different third-party plugins, even (maybe especially) the "demo" versions. CW introduced me to PSP Audio, for example, and Melodyne, and others, which are now part of my, er, "signature sound".
 
4) The people I've encountered in these CW forums and their advice, tips, insights, philosophizing and dust-ups. Goodness golly, what a treasure these posts are (Um, I don't have to include the Coffeehouse in this, do I?), what an influence on my mixing (and a lot of other more-or-less audio/music related topics)! [Swell the Max Steiner score.]

bitman
To get that pop country bass sound that almost sounds like just a constant strong bass tone, I used the phase rotator
in stardust vst then followed that with a limiter and slammed it all.
 
Seems to have got me closer.
 

Man I'd love to hear that!!!!  Anything online to listen to?

As a follow up to my own thread...first, thanks for all who contributed.  I literally made a list of things I need to try/do/get better at as a result of your suggestions.  Two HUGE things that have a made a big difference in my mixes as a direct result of ideas presented here are:
 
1)  Gain staging.  I thought I was doing it right the whole time using Satson Channel and Buss, but was still running way too hot into my master bus.  Fix...TURN THE FREAKING BUSES DOWN.  Solved.
 
2)  Dave Pensado video about M/S stuff, cleaning up low end, and sweetening the mix.  This video is incredible if you haven't seen it.
 
https://www.youtube.com/watch?v=jKnRRrER718
 
Cheers everyone!!!!
 

It's more in general operation terms, but spending time to learn and set shortcut keys immensely sped up my workflow. Also, the five minutes I spent looking at a speed comping tutorial (for X3) have been the single best invested five minutes I've put in the program.
 
For the rest, I'll have to go with some of the obvious ones; highpassing a lot of stuff and parallel compression.