What makes a good Hi Fi rack improve your system.

Vibration feedback and transmission to the chassis. A good rack can reduce these effects and limit it to airborne vibration and feedback. That said, I have wondered why good isolating feet can't get most of this curtailed. And I say most, as I realize the stand and shelf will always be the ultimate preference, but sometimes budget gets a crucial vote.

I agree ,some i attribute to style and some to fuction . Price is also a factor for a mid level guy like myself , i have a steel lead shot filled stand in my 2 channel room my tt is mounted on the wall with a 2" maple base . My ht stand is a salamander , steve blinn are nice too i believe there is a balance which is imo offset by gear . I cant justify 6k for a stand , i have found thick and heavy wins the game just my opinion ..

Great question.  I use the Quadraspire Sunoko Vent and it is tremendous replacing what I assumed was credible, a Target stand.  Turns out the Target muddied things, how weird is that?  The bass is much less bloated, imaging improved, I really don't get it, just know the results.

System: Naim UnitiServe, Naim DAC-V1, Manley Jumbo Shrimp pre, Manley Mahi Mahi monoblocks, Merlin BME monitors, Decware cabling/ICs, custom PCs, Sound Org. Stands, Furman Elite 15A power conditioner.

When I lived in the UK (some 20 years or so now) I was a big fan of the Audiophile Base system. It looks good, is versatile and expandable and is well designed from an isolation standpoint. As I recall it was also good value

Somewhat higher end and no doubt prohibitively expensive as an import in the UK I now use Grand Prix Audio racks

So if you have StillPoints under all of your components you can further improve the sound with a better rack?

Ricred1 - absolutely. Rack design, choice of shelf material and footers all matter. Design of the rack and the material of the shelf will all affect the vibration transmitted to the feet thus definitely has an impact. As a rule solving the problem down the chain (rack-> shelf -> footer) is best but a good footer will always help as it affects the transmission of vibration both to and from the component as well (i.e. Changing the component to shelf connection)

For the GPA racks I use the stock shelf is acrylic, upgrade is carbon fiber and I have one custom shelf under my pre amp in a special composite made by Marigo labs. Them the footers you use make a huge difference, I use Marigo labs F8 extreme mystery feet which really eliminate micro vibrations and work well under everything I have tried them on. Others like Stillpoints but I’ve never tried them

As an example of how tiny things matter in the latest mystery feet update three dots of sticky tape were added to the top of the footer and they materially changed the sound of the footers, like I said all of these things matter. Good news is the effects are all additive and you can stop once you feel you are not getting good results for your spend

If you employ good isolation devices (roller bearing, air bearing, springs), the nature of the underlying surface is of far less significance.

I like movie Avatar. 
If we discover Unobtanium, the components can remain in air.