[Obama's] eloquence is different from what I think of as rhetorical prettiness -- words and phrases that catch your notice as you hear them, and that often can be quoted, remembered, and referred to long afterwards. "Ask not..." from John F. Kennedy. "Blood, toil, tears, and sweat" from Winston Churchill. "Only thing we have to fear is fear itself" from FDR. "I have a dream," from Martin Luther King. Or, to show that memorable language does not necessarily mean elevated thought, "segregation today, segregation tomorrow, segregation forever!" from the early George C. Wallace.
At rare moments in history, language that goes beyond prettiness to beauty is matched with original, serious, difficult thought to produce the political oratory equivalent of Shakespeare. By acclamation Lincoln's Second Inaugural Address is the paramount American achievement of this sort: "With malice toward none, with charity for all, with firmness in the right as God gives us to see the right..."
The reason to distinguish eloquence of thought from prettiness of expression is that the former tells you something important about the speaker, while the latter may or may not do so. Hired assistants can add a fancy phrase, much as gag writers can supply a joke. Not even his greatest admirers considered George W. Bush naturally expressive, but in his most impressive moment, soon after the 9/11 attacks, he delivered a speech full of artful writerly phrases, eg: "Whether we bring our enemies to justice or bring justice to our enemies, justice will be done." Good for him, and good for his staff.
Rhetorical polish, that is, can be a staff-enhanced virtue. The eloquence that comes from original thought is much harder to hire, or to fake. This is the sort of eloquence we've seen from Obama often enough to begin to expect.
(Sorry for the long quote, but I wanted to include all of Fallows's examples.)
Inventio is the system or method used for the discovery of arguments in Western rhetoric and comes from the Latin word, meaning "invention" or "discovery". Inventio is the central, indispensable canon of rhetoric, and traditionally means a systematic search for arguments (Glenn and Goldthwaite 151).
Inventio comes from the Latin invenire, meaning "to find" or "to come upon". The same Latin root later gave us the English word inventor. Invenire is derived from the Greek heuriskein, also meaning "to find out" or "discover" (cf. eureka, "I have found it").