A good quality wall outlet should provide (1) greater surface contact area, like Creeper said, (2) better grip, like Beowulf said, and (3) be physically stronger to resist cracking from strains from heavy cable and from impacts. If the surface area is small, any corrosion or pitting would result in some slight voltage loss, some impedance to easy current flow, or possibly electrical noise from arcing. A better grip is important to keep the plug properly set in the socket to maintain full surface contact (as well as prevent electrical shock from exposed contacts or unintended disconnects). I have had cheap outlets crack from wear and impact from repeated use with vacuum cleaners, portable electric devices, etc. Of these three issues, perhaps the most important is better surface contact, because it is invisible but affects the electronics and could be audible.
