Just as the choice of wine glasses can make a significant difference in the enjoyment of a wine, the cup can have a significant impact on the enjoyment of an espresso. Part of this difference is aesthetic, but part is also very practical.
Because espresso is so sensitive to temperature, a thick-walled ceramic cup (ideally: porcelain) capable of retaining heat can make a big difference to its flavor. The cup should also be preheated for the same reasons that ceramic dinner plates are preheated in gourmet restaurants. Furthermore, a small cup size with a narrow width also provides better support for a layer of crema on top of the espresso. Large cups are detrimental to a good crema.
Many Americans scoff at the thought of drinking beer from a can, drinking wine from a plastic party cup, or eating off paper plates, and yet we don't think twice about drinking our coffee from a paper cup. America's general preference for quantity over quality combined with a "to go" culture unfortunately make paper cups the rule for espresso, and not the exception. You'll never hear the phrase tazza di carta spoken by a customer in an Italian espresso bar - and with good reason. As my Italian teacher would say, "Criminale!"
And as a blog I read once put it, "Coffee is best enjoyed at a table not a steering wheel."