Great slides make for great presentations. But more than that, great slides really help people understand the information you're trying to present to them. Zach Holman outlines the finer points of this in a post on slide design targeted towards developers.

My slides are not designed for people who didn’t see the talk in person. They’re designed to support my words, not some online audience. What’s more, many commented that they found the design of the slides to be noteworthy. I’m expressly not a designer.

You really have to give Zach's post a read, there's a ton of great information in it, and to be honest, I'm loving the design of his slides, the typography really shines.