I found this icon set and was struck by how unique they are yet how they look the same. Finding what was different and the same about them was interesting.
One of the unifying factors are the shadows. I noticed that the objects always casted a distinct shadow to the lower right hand corner and was consistent through each. All are the same except the one in the upper right hand that has a smaller shadow casted by the pin. However, they are the same in that the object that is standing out or raised is casting the shadow.
The most noticeable factor that unifies the icons is the fact that all of them are set inside of squares that have rounded corners. Three of them are simpler objects that fit inside of the box, but the other three go off the edges. They are different yet unified.
The opposite of the shadow is the highlight that is on top of the objects to the left. It creates dimension and a raised illusion.