I just decided this quarter to use slides for my calculus class, a large-lecture course of the sort I'd never done before; I figured it would be easier to see the "board" if it were on the big screen. Here is the progression of my mistakes and corrections:
My first lectures had too many words. Slides are great for presenting the wordy parts of math, because they take so long to write and then the students have to write them again. What is not great about them is how much they encourage this behavior.
Since I was giving a "slide presentation" or a "lecture" rather than a "class", my mindset was different: sort of presentation-to-the-investors rather than gathering-the-children. My slides went by too quickly.
I eventually slowed myself down by basing the lectures around computations rather than information. Beamer is pretty good (though not ideal) for this, because you can uncover each successive part of an equation. If you break down your slides like this, it is almost as natural as writing on the board.
My students themselves actually brought up the point that Terry Tao mentioned in his answer: the slides were too transient. They also wanted printouts. Having to prepare the slides for being printed in "handout" mode changed how I organized them: for one, no computation should be longer than one frame (something I should have realized earlier). Also, there should be minimally complex animations, since you don't see them in the printout.
Many of them expressed the following conservative principle: they had "always" had math taught on the board and preferred the old way. So I've started mixing the board with the slides: I write the statement of the problem on a slide, solve it on the board, and maybe summarize the solution on the slides. This works very well.
Now I can reserve the slides for two things: blocks of text (problem statements, statements of the main topic of the lesson) and pictures. TikZ, of course, does better pictures than I do, especially when I lose my colored chalk.
Preparing these lectures used to take me forever. Using beamer does require that you learn how it wants you to use it: don't recompile compulsively, because each run takes a full minute, and don't do really tricky animations. Every picture takes an extra hour to prepare. If you stick to writing a fairly natural summary of a lesson, broken by lots of \pause's and the occasional \begin{overprint}...\end{overprint} for long bulleted lists, an hour lecture will take about two hours to prepare.