I'll admit that I'm particularly bad at understanding presentations because I have no patience -- "This is boOOooring. Hmmm, what am I having for dinner tonight? Oh crap, what did they just say?" ... and then I'm lost for the rest of the talk. But people, please, at least try to make your talks accessible. Many conferences now have "best paper" awards; I think they should also have "best talk" awards so that everybody tries harder. Related to that: WHATEVER YOU DO, UNDER ANY CIRCUMSTANCES, DO NOT USE BULLETS IN POWERPOINT. ALSO, ALWAYS TURN OFF AUTO-RESIZING OF FONTS. Your fonts should be 28pts or higher, and the size should be uniform throughout the presentation.
If more than 75% of the audience thinks a talk really sucks, we should ban the speaker from that conference for the next three years. After three such infractions, the speaker should be voted off the research island.