MarketingProfs.com has just published my new article, "How Web Widgets Help with Viral Marketing". Don't hesitate to post your questions or comments here. And please feel free to send it to your friends and colleagues who might find it interesting.
Oh, and thanks to Brandon for proofreading the article and suggesting edits.
In the case of YouTube, the widget actually solves the #1 problem. MPEGs, AVIs, and QuickTime movies have existed for years. What was not straightforward was a non-HTML programmer quickly embedding the video in her web page. YouTube's widget made it easy. So, in retrospect, it's not too surprising that it was a big success.