Online video definitely existed before YouTube came into vogue. However, uploading videos, sharing and watching them was quite cumbersome. (...) Video files were too large to be e-mailed. (...) Viewers would typically need to wait for the entire video to download before they could start watching it. This was a problem not limited to just peer-to-peer video sharing. Most professional websites with video content had the same issue. Downloading the video was just half the battle. Users needed to install the appropriate video player, the free versions of which often behaved like "spyware". (...)
Distributing popular and hard-to-find video clips was clearly a success factor. Clips of the popular, long-running television show, Saturday Night Live was a particularly significant example. (...)
YouTube allowed users to easily embed any hosted videos on web pages or blogs. This turned out to be particularly popular with social-networking websites, especially MySpace. (...)
While the technology platform used by YouTube was not particularly remarkable, it was designed to solve the problem at hand. The technology concept was to encode videos in the Macromedia Flash format and take advantage of the millions of computers which already had the Flash player installed on it.
YouTube was launched when social networks started to become popular, when the bandwidth became cheaper and people wanted an interactive alternative to TV. But other sites did similar things, but couldn't have YouTube's success. I wonder why.
{ via Greg Linden. }