and it is an art. i had no idea, but there's so many components to it, and if it's the only output you have in your profession, you're pretty much going to go to extreme lengths to make sure it's as beautiful as possible.
There has been books written on the subject, so no boring details on the matter, just a short story ;)
So in addition to charts and diagrams and tables and all that other fun stuff, we actually put content on our slides (occasionally). well, we actually start with something that could be termed more of a skeleton and then over time 'flesh it out.' This process (as i now know) is referred to as giving something 'more color.'
When I was first learning about these terms and nuances, one of my guides called me over and told me my slide had good content, flowed pretty well, and was aesthetically pleasing, but needed 'more color.' So i headed back to my seat and spent the next hour banging out verbiage so that I could provide more detail. After looking over the new version, i'm told that i sound too 'consultanty' and need to pull back on the flowery language. Next time i'm asked for 'more color', i think i'll just print the slide out and go to town with my crayolas ;)
On an unrelated note, wanted to point you a recent Karmatube video highlighting a friend of a friend, Robert Eggers, who lives the work we got to try out for a bit when we blogged about serving on the streets...