Something practical I haven't seen mentioned yet: unless you know the venue, assume the screen is far away from the audience (so the text needs to be large) and that it's hanging low (so most people can't see the bottom of the slides).
That depends (a lot) on the displaying and viewing conditions; are your slides projected in a dimly-lit room, or are the lights on? Readability of your slides is affected by that.
Although slides often don't contain much information that needs to be read in full detail, I find code examples to be better readable in a dark font on a lighter background.
Sometimes the conference gives some advice regarding the slides (dark text on light backgrounds or vice versa), since they'll know / set the room lighting conditions.
