There seem to be certain features in common. When speech is described as 'eloquent', I think of it as being:
•fluent – it flows easily and at a good pace, without hesitations, linguistic errors, repetitions, or uncertainty in the use of vocabulary, grammar, and pronunciation;
•personal – it expresses, or appears to express, the convictions of the speaker, whose personality comes across in the choice of language;
•appropriate – it suits the situation the speaker is in, or at least (thinking of the final example) it's an understandable reaction to it;
•heightened – it displays features of artistry that go beyond the linguistic norms we encounter in everyday informal conversation;
•clear – it uses words that are known to the listeners, and puts them into sentences in a way that is easy to understand;
•memorable – it contains elements that stick in the mind, so that if asked, 'what did X say?' it's possible for a listener to repeat tiny bits of it (or, in such scenarios as thumb-hammering, a polite paraphrase of it);
•reactive – it shows awareness of the interest levels and listening abilities of the audience, and responds or adapts to any feedback.#6408•