Peter Tatchell (@PeterTatchell) explained to Simon Armitage in the BBC 4 broadcast Speeches that Shook the World (view here) on what makes a good speech:
"I think a good speech has an element of provocation, controversy and even confrontation. Because that's the way you challenge orthodoxy. That's the way you get people to sit up and notice."He also said on the power of language:
"Emotive words can change the course of history; but powerful language in the wrong mouths can be extraordinarily dangerous."