Slightly disjointed observations, inspired by a few presentations I've observed recently:
1. Obvious laziness is unprofessional. I saw a presentation to an audience that works with mathematics where the presenter used the "draw ellipse segment" tool to draw "exponentials" on a slide about exponential growth. Since exponentials look very different from quarter-ellipses, it was obvious that the presenter didn't think the presentation worth taking the one minute required to plot an actual exponential with a spreadsheet.
2. When in doubt, use less: colors, fonts, indent levels, bundled clipart; in fact, never use bundled clipart. Everyone has that same clipart, so the audience will be familiar with it, associating it with the other uses.
3. There is no correlation between the time it takes to make a slide and the time that slide should take in a presentation. I have several slides that took hours to make (just to make the slide, not to figure out the material going into it) that get shown for seconds in a presentation, because that's their job in that presentation. On the other hand I routinely keep one-word chyrons up for minutes, as chorus to what I'm saying.
4. If you're going to use quotations, make darn sure you get the reference right. Otherwise you'll sound like an idiot. Saying "Life is but a walking shadow" and attributing it to 'Q' in episode one of Star Trek The Next Generation is both ignorant of the quotation (Shakespeare, Macbeth, Act 5, Scene 5 – you can find that on the interwebs) and Star Trek TNG where John de Lancie (Q) clearly attributes it to Shakespeare. Also, complete sourcing (not just author) increases credibility by making the quotation easier to check.
5. Speaker notes are perfectly acceptable; just don't carry flash cards. Memorizing a speech is really hard and few people can do it correctly; if you're over 40 you can always make the joke that memory is the first thing to go (punch line: "I forgot where I heard that"). Your command of field knowledge can be demonstrated in the question-and-answer period; coincidentally, people who are good at memorizing speeches tend to do poorly in the Q&A... Just remember:
6. Speaker notes are for the speaker. Don't impose them on the audience. Most especially don't put them in outline form on your slides. It suggests that you don't know how to use "presenter screen" on your computer, or dead-tree-ware. Don Norman writes about that.
7. Preparation is essential. I already wrote 3500 words on this. Most presentations continue to fail due to obvious lack of preparation or of preparation time spent on the wrong end of the process (memorizing speech, rehearsing delivery; these are important finishing touches, but not where most preparation should focus).
And a bonus meta-observation, from Illka Kokkarinen: the biggest problem is still the incessant yammering for fifteen minutes to reach a conclusion that could have been written in one paragraph to be read in less than a minute. Good point! We are so used to our time being wasted that we no longer notice this.
[Added May 30, 2011.] A reader (who asked for anonymity) emails: Don't eat a beef and bean burrito in the two hours prior to the presentation. I'd go further and suggest carefully managing pre-presentation intake of liquids (a presenter with a full bladder becomes short-tempered and rushed) and foods with gastrointestinal disruption potential.