Discovery is the biggest problem for any site with massive amount of content. And it doesn't get any more massive than YouTube, hours of video are uploaded every minute and one billion people watch at least one video every month.
With so many people and so many videos, it's hard to serve the right content to the right users. Still, YouTube's recommendations are getting better all the time.
In fact, YouTube is confident in its recommendations that it's experimenting with an "I'm feeling lucky" feature similar to the search button.
If you visit the youtube.com/feeling_lucky page, while logged in, you'll get a playlist of 10 videos generated based on your viewing habits.
YouTube is also experimenting with a "Play" button, visible on every page, which links to the I'm feeling lucky feature.
You can enable the experiment for yourself to check it out, just follow the instructions on the Google Operating System blog.