This (formal vs informal) is somewhat confusing for me too. I remember listening to a speech by President Ronald Reagan on Voice of America in 1984. He ended the speech by saying "you ain't seen nothing yet" (or perhaps "you ain't seen nothin' yet"). I was very surprised because to me a presidential speech is very formal and this phrase seemed to be far from formal.
The speech is now on youtube. The phrase occurs at the end (22:20 - 22:25).
The last sentence of his speech was - “
We can say to the world and pledge to our children, America’s best days lie ahead, and you ain’t seen nothin’ yet.” Use of this informal phrase at the end of a sentence with a formal word such as "pledge" seemed incongruous to me. But I am sure he
struck a chord with the audience. ;-)
I agree with emsr2d2, with the addition that,
this is a device used in writing and speaking whereby you set your audience up to expect a formal ending to the sentence, because all the rest has been so formal. It's done for contrast and to give a mild surprise or shock to the crowd, who are probably so bored of hearing the same old speeches.
However, it is a very risky strategy to mix formal with very informal, or even slang, unless you know what you are doing. Even then, you take a risk by using it. At the wrong time and in the wrong circumstances it could fall flat on its face. His supporters probably thought it was brilliant and inspirational, while his opponents probably thought it was cringe-makingly awful, and corny. This is why it is risky to use it the technique, because you can divide opinion, some will think it's alright, others not. This is why using the formal or semi-formal are acceptable to most people, in most ordinary settings, unless you absolutely know that informal use is ok. Even then, in an informal setting, you wouldn't normally mix the informal and the formal.
I expect he had the speech tested multiple times on his closest supporters before finally broadcasting it.......because sometimes things like this can fall flat even if you are only speaking to your own supporters.