Knowing your audience is probably both the hardest and most important aspect of giving a great talk. JohnMont (of Popfly fame) just wrote a post on The Art of the Demo TV commercials have it right - you have a VERY small window to get your point across and then get off. Boredom = public enemy #1. As such, max attention span is the most important aspect of any audience to understand. Having given my share of lectures and demos on campus here's what I've found (in odd numbers):
Students outside a talk = 30 seconds Students in a talk = 3 minutes Students being quizzed later = 30 minutes
Aside from that more technical = more questions :)
I also liked Alfred's note that teachers demo every day in his reply. With that in mind, maybe I need to get back to school...