I attend many presentations and have seen many that had a lower quality than they should have. Some things are difficult to change (for example I have difficulty speaking slowly). But there are some things that are easy to change that many people seem to get wrong and I will list some that stand out to me.
Unreadable presentation notes. You have to use a reasonably large font for it to be read by most people in the room. This means probably a maximum of about 16 lines of text on the screen. I have attended some presentations where I couldn’t read the text from the middle of the room!
Too many slides. On a few occasions I have heard people boasting about how many slides they are going to use. An average of more than one slide per minute does not mean that you have done a good talk, it may mean the exact opposite. One of my recent talks had 8 slides of main content plus an introductory slide while waiting for people to arrive and a Q/A slide with my email address and some URLs for the end. The speaking slot was 30 minutes giving an average of a slide every 3-4 minutes.
Paging through slides too quickly. If you have 60 slides for a one hour talk then you will have no possibility of going through them at a reasonable speed (see above). Even if you have a reasonable number of slides you may go through some of them too quickly. On one occasion a presentation included a slide with text that was too small to read, I tried to count the lines of text but only got to 30 before the presenter went to the next slide.
Using slides as reading material for after the lecture. Sure it can be useful for people to review your notes after the lecture, and it’s generally better to give them the notes than to have them be so busy writing notes that they miss somehting you say. But if you want to have something verbose and detailed that can’t be spoken about in the lecture then the thing to do is to write a paper for the delegates to read. Serious conferences have papers that they publish (minimum length is generally 4 solid A4 pages) which are presented by a talk of 30 to 60 minutes. That way people get a talk as an introduction and they get some serious reference material if they want to know more. Also people who miss the talk can read the paper and get much of the value. Is it not possible for slides to take the place of a paper.
Bad diagrams. Diagrams should be really simple (see the paragraph about readable text). It is OK to have diagrams that don’t stand alone and need to be described, a lecture is primarily about talking not showing pictures.
When simplifying diagrams make sure that they still represent what actually happens. Simplifying diagrams such that they don’t match what you are talking about doesn’t help.
Animations. The only thing that is animated in the front of the room should be the person giving the presentation. Otherwise just do the entire thing in flash, publish it on the web, and don’t bother giving a talk.
Staged content, particularly when used as a surprise. Having a line of text appear with every click of the mouse forces the audience to stay with you every step of the way. This may work for primary school students but does not work for an intelligent audience. Give them a screen full at a time and let them read it in any order that they like. This is worst when they someone tries to surprise the audience with a punchy line at the end of every paragraph. Surprising the audience once per talk is difficult. Trying to do it every paragraph is just annoying.
One final tip that isn’t as serious but is not obvious enough to deserve a mention. Use black text on a white background, this gives good contrast that can be seen regardless of color-blindness and with the bright background the room is lit up even if all the lights are off. The audience wants to see you and sometimes this is only possible by projector light. Also the more light that comes out of the projector the less heat that builds up inside, it can really mess up a presentation if the projector overheats.