What are the STOL features on the Pilatus PC-6 Porter?
Quite simple.  Big wings with decent flaps, lots of power. 

11.2 lb/hp at gross.  Wing loading of 19 lb/sf.

Comes down to how slow can you fly, and how fast you can get to that speed.  Without exotic high lift devices, it's mostly about wing loading and power loading.  The Pilatus only has to accelerate to a little over 60 mph and there is power to get it there in around 600 feet.  So there you go.

It's STOL, but not what I would call "super STOL".  For that you'd need full span flaps and slats and or bigger wings and a higher power to weight.  Like a Helio Stallion.

The Porter isn't a unique super-plane and it doesn't have any fancy high lift secrets.  It still needs about an 800 ft runway.  It just has a good all around configuration for its job, easy to load, and is simple and rugged.  It's the simple and rugged + good-enough-for-most-short-strips short field performance that's made it successful.