YouTube is constantly improving with updates and new features getting added. The latest addition “auto-complete” feature that shows query suggestions as you start typing characters in the search box. YouTube Suggest has its own list of queries obtained from YouTube users, so it should offer decent suggestions. The feature is enabled by default, but you can disable it in the "Settings". For now, YouTube Suggest seems to be live only for international sites like YouTube UK and only if you search from the homepage, but it should be available at YouTube's main site in the near future.
You'll notice the similarity between this and Google Suggest, a project that is about to finally graduate from Google Labs. “By suggesting more refined searches up front, Google Suggest can make your searches more convenient and efficient by keeping you from having to reformulate your query. Google Suggest might offer suggestions that you will find novel or intriguing,” explains Google FAQ.
Google, YouTube, Online Video, Internet Video, Video Sharing, Auto-suggest, New Features, Search Queries