Good speakers are not afraid to use simple language. They have confidence in their ideas and don't feel they need to couch them in flowery or pretentious language. Pretentious language draws attention to itself and away from the idea it is supposed to express. This doesn't mean that the language of a speech ought to be simplistic or that every speaker should talk like Dick and Jane. The language in the examples I've cited is simple, but it's far from simplistic.
Here are some suggestions for making language work for you when you write or deliver a speech.