What is it about overhead projectors that causes us to become lousy communicators? Why do our speeches or presentations lose much of their steam when we use overheads?
Well, for starters, we often give more attention to overheads than audience. It can't be helped. After all, we have to pick up right overhead, separate it from next one, get it on projector properly, check it out on screen, and so on.
While doing those things, we're taking our eyes off audience. At same time, audience spends a lot of time looking at screen, rather than at us. And, nothing detracts from good communication like loss of eye contact with audience.
We're also taking our mind off audience. Instead, we're focusing our thoughts on technical issues involved in showing overhead, including our explanations of visuals.
Then there's amount of material. Almost every time I see a speech with overheads, I see way too much content. One of best lessons I've learned in several years at Toastmasters is that less is more. Don't try to explain everything to your audience, just pick one small sliver of an issue and explain it well - a speech is not a book or a written article!
And, then there's simple fact that projector gets between you and audience. There's noise and size of projector, which mean a projector can be a more powerful presence at front of room than you.