WordPress core supports auto-embedding videos from Youtube and other video site links. It also understands the messier links that can be copy-pasted from the browser's address bar, such as
I will add support for this soon. Embedding other messy types of links like
http://www.youtube.com/user/[user]?feature=watch#p/a/u/0/[videoid] isn't supported by WordPress core. Please let me know if a plugin allows embedding videos with link types that are not yet supported by Video Thumbnails and I'll include support for them in my next patch.