Delivering a strong speech depends on a number of things. Here are five elements that I think are very important:
1) Insight. Your audience expects you to be well-informed on your topic. You need to be general enough to get your ideas across, but show that you have an insight that no-one else has come up with.
2) Analogies/Parallels. You need to be able to demonstrate how a current situation relates to one that has gone before. This makes it much easier for your audience to understand.
3) Evidence. It's no good making statements that you can't justify. You need to provide examples to support your argument, rather than to make your argument.
4) Endorsement. This is like calling an expert witness in a court case. If you can cite (other) acknowledged experts who agree with you, it makes your case much stronger.
5) Humour. This is not (definitely not) joke-telling. Leave that to the stand-up comedians. Your humour should be natural and in context with the speech.
If you can combine all those five elements, your speech will be strong and purposeful. All you need to do then is to deliver it well.