Express rather than impress. This saying is probably part of a journalism class 101. This means that the goal is to be able to convey what a person wants to convey and is not so that the listeners will be wowed by the big words and complex metaphors and analogies that the speaker will use.
To express means giving the listeners the chance to understand the speaker is saying. Thus, public speaking is not about the speaker, rater, it is about the listeners. The ultimate of whether a speech is a success or not is the fact that the listeners were able to understand what the speaker said or is saying.
No matter how great a speech is, regardless of how impressive it sounds, if the listeners did not understand the message, the speech is a failure. Having said this, there are some points to consider in making a good speech, to wit: Words, audience and context.
Choosing the right words is important in order to convey the message clearly, but is should not also be to the point that the audience will be insulted with the too elementary choice of words. There is a difference between using the right language and using an elementary language. The former, that is using the right language, means using the best word for a particular. The best word could either be a very simple word or a very technical or unusual word. But the former is not always the better choice just because the goal is to express. If the latter would better represent the idea, then it is the best word to use for the speech.
Public speaking is not about being very good in the language used for the speech. It is not about being an expert in the topic, although this is also a very important factor when delivering a speech that requires credibility and authority. It is not about being good at grammar, although being grammatically incorrect might mess a speech. The point is, it is not all about these things ... It is about using the most appropriate word and language based on the kind of listeners one has.
Having said this, who the audience is plays a very important role in a speakers speech. It is not about the speaker. It is about the audience. This is because of the bottom line that public speaking is meant to express and not to impress. The recipient of the speech is what is important. If they understood the speech, then it is a success, more or less, If the did not, it is definitely a failure and the time spent by the speaker for public speaking was just waste of everyone's time.
Context is also very important because the speech should always be in keeping with the other pressing issues and concerns in the surroundings.