I'm continuously baffled by how little effort scientists put into public presentations. It's easy to downplay the importance of talks when there are so many other constraints on our time, but, we need to take into consideration the sheer amount of collective time that bad talks are wasting.
It's odd that there seem to be no incentives to improve the quality, or most importantly, the timing of talks. For instance, I can't count the number of talks that I've attended where the speaker's gone way over time . Conversely, I can count the number of times that I've seen someone ask the speaker to stop on one or two fingers. Scientists have never struck me as sheepish about offending colleagues' feelings when it comes to reviewing papers or criticizing work during lab meetings. And yet there seems to be some kind of universal ban on offending folks, or even providing constructive criticism regarding presentations.
I wish that it would become culturally accepted that unnecessarily long or uninformative presentations waste the precious time of every single attendee. It should be acceptable and expected that a moderator first give a speaker a signal that their time is coming up (a five-minute warning, for example), and then politely cut them off when that time arrives. I have a feeling that people would feel embarrassed to be cut off, which would provide at least some incentive to do a better job putting together their talks .
Here's a general observation: despite over a decade in the 'biz', I have never heard a seminar attendee describe a presentation as 'too short', while the converse is as regular as clockwork. This is probably an excellent indication upon which side to err when prepping a presentation.
Finally, I'd be remiss not to bring up two personal pet-peeves about seminars:
1) I've noticed a trend towards a particular presentation style that I call the 'look how much work I've done!!!'-talk. This is where the speaker focuses on telling you about the effort they've put into something, usually by presenting a lot of slides without going into detail about any of them. In my experience, this is always a bad idea. It's much better to focus on one aspect of a project in sufficient detail to convey why it's important, and why people should care - both of which are rarely as self-evident as people would like to think.
2) The purpose of overview slides are to help the audience put the various parts of a talk into context. However, I notice that most people use them as a long-winded abstract. On top of taking up valuable time, I don't find it helpful to receive a barrage of concepts all at once, before they're properly explained. Again, I'd focus more on why it's important, and why people should care at this point. Also, I don't think that any talk shorter than 30 mins needs a minute-long overview slide .
P.S. I think that these concepts should apply to all talks - not just big, public seminars. No need to waste time polishing lab meeting presentations, but it's no less important to be considerate of your audience's time.
 I've actually been to a conference where our entire session had to miss dinner because we we're so ridiculously behind schedule.
 While practicing a talk before the official delivery is ideal, I don't think that this is required. With a bit of experience, you can develop pretty reliable rules-of-thumb about how long you should spend on a each background or data-heavy slide and so on.
 I know that a lot of people disagree with me on overview slides, but I've seen so many talks begin with a 'First I'm going to give you an introduction to X. Then I'll talk about some of the results I've obtained, before discussing their implications. Finally, I'll end with some conclusions'-slide. I don't think that we need to be reminded of how a talk works. If it's not helpful, it's unnecessary.