My personal preference as a speaker and a listener is for the minimal: none of the stuff at the bottom or the top, and no navigation icons (those things at the bottom-right of the screen). Thus, if a slide contains no text, it's a totally blank white rectangle.

Enough people have asked me how to do this that I put instructions on a web page.

(You'll see there that my opinions about the Beamer manual aren't entirely the same as Andrew's.)

Also, I think it's helpful to use colours sparingly and systematically. E.g. it can help to put "Definition:", "Theorem:", "Lemma:" all in blue (say), but putting "Definition:" in blue, "Theorem:" in red, and "Lemma:" in pink is just distracting.