One of the most interesting lectures that I've heard over the past perhaps five years was presented without any audio-visual aids. The lecturer simply described a program which she had initiated, and the program itself, and the details she described, were enough to convince those listening (or at least me) that she was conducting a wonderful program.
But this is, of course, the exception. Whether it's a concert, a play, a sporting event, or a lecture the audience comes with certain expectations, and it's best not to play with those too much. In lectures, we expect to be shown slides that supposedly explain and/or expand upon the material being discussed. All too often what we get instead is slides that present to us, printed out word for word, the same thing that the lecturer is telling us. Slides of this sort are more aids for the lecturer to remember what he or she wanted to say next than they are to those attending the lecture, but often much of the audience feels slighted if materials of this sort aren't used.