We take Johannes's work as an example but he's one of the best in the business and even his work really stands off.
If we are trying to evaluate render engine realism it's only natural to compare real photos with the work of the best artists in the industry. The strange thing is that I saw the real kitchen photo I posted many months ago and it suddenly popped into my head while I was looking at Johaness' work. That's why I posted his work but in all fairness I should have posted another similar interior from (insert your favourite CGI artist) who uses Corona. If I take the best Corona image I can find and compare it to the two images I posted I can clearly see that Fstorm's render looks more photorealistic. Even when comparing it with the latest work from Jakub Cech which is drop dead gorgeous.
With regards to Fstorm the only way I can explain it is as if the materials have more "depth" along with what was already said about better tonemapping. In my opinion Fstorm users have the advantage of creating better looking materials because of the built-in tonemapping which gives them the ability to see materials how a camera would see it. In Corona you have to fiddle around with the sliders and LUTs until you feel satisfied and go on from there. Some Corona users get better at this, some master it, some don't and some never will.
I'm not trying to bash Corona because it's a great render engine and it got me excited about CGI again but as with all things there is always room for improvement.