What are some bad effects of bullets in presentations? I get confused with some comments of JeffE on this question. Those are:


  ...avoid bullets entirely.
  
  Powerpoint doesn't kill people. Bullets kill people.


The comments got a lot of upvotes, so there are many people agree with them. And I don't understand why.

I use block and itemize extensively, and I think they provide a clear structure to my presentation. The bullets are the key points that I want to present on that slide.

So why people think they are so bad?

I have to emphasize that the question above asked about teaching, while I only give presentations. Are there any differences?



UPDATE

Thank you for the excellent answers (I still want to hear from JeffE). I can kind of understand that using only bullets is not enough, but avoiding them entirely is a personal opinion, not a general rule.

By the way, the best presentation I've seen so far is the last lecture of Randy Pausch, in which he used a fair amount of bullets.
TL;DR: A bulleted list onlys show one kind of relationship (either sequence or membership in a set). If you want to show another kind of relationship, it's better to use something else. If you want to show sequence/membership AND something else, you can either use a different visual tool (not a list) or a list + additional dimensions.



Bullets are useful to help speakers remember what they wanted to talk about, but they're not a powerful way to visually convey ideas and relationships between ideas.

To quote Edward Tufte in The Cognitive Style of Powerpoint:


  By leaving out the narrative between the points, the bullet outline
  ignores and conceals the causal assumptions and analytic structure of the
  reasoning.


He in turn quotes the Harvard Business Review:


  Bullets leave critical relationships unspecified. Lists can communicate
  only three logical relationships: sequence (first to last in time); priority (least to most important or vice versa); or simple membership in a set (these items relate to one another in some way, but the nature of that  relationship remains unstated). And a list can show only one of those relationships at a time.


Bullets may be OK when you want to convey sequence or membership in a set. But even then, lists often have more than one dimension, in which case there are almost certainly better ways to present this than just as a plain text list with bullets. For example, take this slide showing an outline of a talk by Tom Rondeau:



and compare it to the classic way to show an outline with a list:



The point of this example is to show that the "plain" list, which is a fairly standard way to show an outline, conveys one dimension. The list version conveys sequence, which bullets can do fairly well (the parenthesis attempt to add another dimension, but it's subtle). The graphical version shows the sequence of the talk and also adds a color dimension to show the content of each section and its classification. 

You could of course add extra dimensions to the list, too. This image was contributed by Eddie:



But if you are just listing points and not trying to show order, priority, or membership, all the more reason to stay away from lists.

Also see Bullets versus sentences in posters (I hate bullets in posters).