Primates have many unique qualities, even among other mammals, including larger brains and opposable thumbs, according to BiologyReference.com. Both of these adaptations allow primates to interact with their surroundings in different ways than other mammals.
Larger brains allow primates to think things through, to rationalize. In most other mammals, actions are instinctual. Actions are not determined by the animals, but by chemical reactions within their bodies. In primates, their more complex brains allow them to process information about their environments and make decisions based on this information.
Opposable thumbs allow primates to grasp and pick up objects. This is one of the most noticeable differences between primates and other mammals. The ability to grasp and hold is the reason humans, and some other primates, can write, paint and play piano, among other things. Without opposable thumbs, most human actions would be nearly impossible.
A few other attributes, highlighted on BiologyReference.com, that make primates unique are that they rely on sight over smell and they have a focus on group and social behavior. Primates use vision to interact much more often than smell, and because of that, their eyes are suited for color vision and are capable of a greater depth of field. Primates are often found in social groups, and in general, it is odd for a primate to live a solitary life.