Presentations shared online are different from the "classical" setup of a presenter giving a live talk backed up by some visuals. Online presentations need to stand on their own, without verbal explanation, and need to be able to attract viewers without the help of "please sit down, the presentation will start now".
Seth Godin posted some good thoughts about what makes an effective book cover. Jeff Bailey asked the question whether this only applies to books. The answer is no. Especially when your presentation has to stand out in SlideShare or on the notice board near the university coffee machine.