When discussing the future of robotic and artificial intelligent technology, C-3PO and R2-D2 are often used as an attempt to illustrate the choice between designing humanoid robots and non-humanoid robots. However, these comparisons are often superficial and overlook the fact that C-3PO's humanoid design and R2-D2's non-humanoid design is very much tied to their function rather then arbitrary design decisions.
C-3PO exists as a translator or liaison between people, droids and maybe equipment of difference species and makes. For example, he translated what Jabba the Hut said to Luke in Return of the Jedi and often translates R2-D2's beeps. His knowledge of six million forms of communication means he is much better at this then a person could be in the same position and having a humanoid form ensures that there is no sacrifice in usability by using a droid. If C-3PO looked like R2-D2 then the conversations would be more awkward as the addition of a droid would be much more obvious.
R2-D2 exists as a replacement for a humanoid engineer and thus he is essentially an intelligent toolbox. His design and unique non-humanoid features reflect and support this role including the ability to conduct maintenance with a variety of built in tools and the ability to do his job while rolling around on the hull of ship in space. A versatile droid such as R2-D2 would be difficult to create if he looked like C-3PO. In addition to the practical issues of putting R2-D2's features on a humanoid form consider the confusion that would happen in a frantic battle where crew members are getting blowing into space and a medical droid sees a humanoid form clinging to the side of a ship.
The reason that C-3PO looks humanoid while R2-D2 looks like a rolling toolbox is not arbitrary or due to superficial reasons such as friendliness. Their respective forms are required to provide the best user experience and ensure that they are each fulfilling their unique purpose.
