How do you know what you're missing?

Read the graphs (frequency response, waterfalls, polars). Then compare them to other speakers.

In today’s market, where it’s almost impossible to hear speakers, much less in a good environment, I wouldn’t buy a critical-listening speaker if I couldn’t see the graphs, or that weren’t recommended by a speaker designer that approaches design from this technical perspective (planars and omni-directionals excluded). Subjective reviews are completely useless to me.

Cat, the sound of any system is a moving target but it does come down to what you hear. Imaging and sound stage are easy to qualify, tonal balance is not. Without a "calibrated" reference it is impossible to know what you are listening to. It is not at all uncommon to see people prefer systems that are way out of whack. A good example is if I increase a systems treble 3 to 6 db over 4 kHz many if not most audiophiles will think this is better. Distortion over a certain percentage is rather obvious and dysphoric. 
This is where good headphones come in. If you are not sure where you are listen back and forth between your headphones and the system. Ideally the tonal balance should be the same at the listening position. 
Describe what you hear as an image in a diary with three program sources that you know inside out. 
Listen to as many systems as you can. The best systems will float an image in space giving you a sense of three dimensions most systems will only give you two dimensions. 
Having said all this, your system is your system and if you prefer it a certain way that is your prerogative.
Others have mentioned comparing a system with live acoustic instruments. How are you going to play live acoustic instruments through your system? Those instruments are going to be affected by the room they are playing in, a situation you have no way of replicating.  
Good systems are always an evolutionary process. So you listen to as many systems as you can. Keeping a diary of your experience is always helpful. The system you liked the best is the one you start to emulate. 

Really an excellent question.  One that I’m asking myself just now.
Being a former musician, I know very much what live music sounds like.
And my system does resemble it closely.
But I didn’t spend real mega bucks [<$20,000) on it. What do systems in the upper five figures and above sound like?  How real is real?
After a certain point are we just chasing minute changes that really don’t matter?

You won’t know what you’re listening for until you hear it.
Upgrade your most "suspect" cable and sit back. It’ll take 2 weeks or more for it to break in and you need to hear what that sounds like. What your favorite vocalist sounds like as break in takes place. Then do it again.

Almost all audio system are never listened to their potential maximum S.Q. by their owner that will never know what they are able to give....

Simple, learn to embed your audio system in the mechanical dimension, in the electrical noise floor of your house, and in the acoustical dimension of the room...

After that most upgrade would become less tentative and often useless if your audio system parts were wisely choosen to begin with...

Upgrading is half the time a reflection of our ignorance about our actual own audio system...

My system value is 500 bucks, upgrading it will cost me too much (more than 10.000) because it is so well embedded now, and anyway i am now happy with it....

Are you able to listen to 9th Bruckner symphony and able to hears all orchestral details and instruments timbre? If yes, your audio system is well embedded and wisely choosen....

Audiophile experience may cost peanuts if you learn how to embed your system.....