Have semantic markup for typing equations. It probably depends on your field, or the target audience, but on some of my slides about half of the content is equations. Defining macros that explain the meaning rather than the syntax of my math formulas helps to keep the source readable and often also concise. (And this is not only true for slides but for any kind of document!)
Use the itemize and "block environments" for pretty much all the text I need. You don't really need any bloat of markup for these.
Don't abuse overlays. Probably most of the code and "hacks" you find in the sources of a beamer presentation are because of this. I tend to use overlays only sporadically, perhaps twice, definitely not more than five times in the whole presentation. Some advice I once overheard about making slides: "Don't play striptease with your audience."
Code for TikZ figures go outside in their own picture.tex which is then \input-ed at the appropriate place.
And define macros. For the ocasional times when I need to do some "hack" that will be needed in many places all over the presentation (for example once I wanted to have some particular formulas display with some color and framed by a box) I would define, of course, a macro just once to do the dirty work, and then use the macro everywhere I need in the slides.
PDF is not just designed for printing. It is a format for displaying electronic documents, independently of hardware (printer, screen, e-reader, smartphone, projector etc.) and software (such as the operating system). This format is both good for printouts and presentation, so a good choice as output format.
Presentations with technical or scientific content are often based on documents, very often written in LaTeX also because of it's mathematical typesetting capabilities. It's very natural to use the same tool, LaTeX, for the presentation.
The benefits of LaTeX, such as separating the form/style and the content, portability in source, implementation and output, cross-referencing capabilities and typesetting quality, are great for presentations as well.
As I use LaTeX, I can work on the source using Windows, Linux, or Mac OS X, and I can give the presentation away as a PDF for reading or presenting on a conference, the reader or speaker can use Windows, Linux, Mac OS X, a tablet computer, a smartphone - or a printout.
If I would use PowerPoint - well, I would be pretty bound to Windows Version X with PowerPoint Version Y or a suited viewer, which is also a limit for the reader or presenter.