What are the modifications on this Kossel?
Cool you placed my Kossel Mini on Stackexchange!
What I have modified to the standard kit:


Retractable probe using RC Marlin (Rich Cattels Marlin with auto bed probe functionality). Never have to adjust endstops after it is once setup.
All can be done in firmware.
Made a transport case, in the picture the cover is missing which is a triangular case which fits over the printer.
You can clip it on and the carry it with the handle on top.
I just throw it in the car and bring it to the Hackerspace and Hackerevents/Makerfaires and when I feel to it, I can do an auto-probe to align it again (which is hardly needed BTW)
Another mod is the Vaeder extruder, which is really cool in the way it uses a GT2 timing belt to drive the filament. It uses a round wheel and it drives the filament over 1/4 of the diameter of the wheel, so there is a 3 to 4&nbsp;cm area where the belt drives the filament.


Advantages: 


Lots of retracts don't "eat" into the filament causing feed issues
Doesn't flatten the filament. Keeps it nice and round.


Disadvantages (at this moment):


Not possible/ very hard to do filament swaps on the fly (to experiment with multicolor prints).
First time assembly can be a head ache, once you have it assembled it really works and is easy going.


And last but not least. I made a foldable Roll holder.

It is mounted on top by two M8 screws with knobs. When I am done I undo these two screws, fold in the roll holder and it is then mounted to the side of the flight case. This way I can transport printer and roll holder carrying it only using 1 hand. So I use the other hand carrying the bag with accessories like rolls of filament, 3D spray, tweezers etc.

Maybe this info helps other people new to 3D printing improve their printers, or enables them to bring printers more easily to meets and events to inspire others to build more Delta printers. As it's just mesmerizing to see them print. I love watching the print come to reality because it such beautiful piece of engineering!