"YouTube co-founder Jawed Karim on How YouTube Got Viral.
1) related video recommendations 2) one-click emailing to spam a friend about a video 3) more social networking and user interaction tools like video comments 4) an external video player
Re: #4, the external video player did have an amazing impact. In a matter of months, YouTube seemed to go from nowhere to everywhere due to that slick and easy to embed player."