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...