The other day I was on my way to Hilton Head South Carolina. I boarded the airline which was on time. I sat back in my seat awaiting the usual safety drill. The attendant ended the announcement with "This is a no smoking no complaining flight. If you complain you'll be the entertainment - outside gone with the wind."
There was some knee jerk laughter and then the message set in. Ouch! The real message was "Don't mess with us. We're not putting up with nonsense." Not exactly warm and welcoming.
What kind of tone do you set when you begin your presentation? Do people feel that you're glad to be there? Humor is a great ice-breaker when used appropriately. But when used to couch a threat the atmosphere can quickly turn negative.
By all means let the audience know your expectations and use humor to engage and drive home the learning points. Otherwise, your message could end up blowing in the wind.