The first thing I noticed about this set of icons was its composition. All of the icons are arranged in such a way that they all remain pretty consistent in proximity from one another, yet they all come together to form a circle, leaving white space all around.
This designer also uses fills instead of strokes in this design. He keeps that rule consistently throughout the entire icon set. Leaving strokes off of the edges of the shapes gives the images a more realistic look.
I really think that this designer’s use of a single light source is what brings the icon set together the most. If you look at the shading on each icon, you can see that all of the light is coming from the upper right-hand corner of the design. The designer’s use of highlights and shadows gives depth and continuity to the icon set.