Ok we have goofus, now I'll be gallent:
1. Complement the poster if you liked the video.
2. Write a comment to the audience in general, not the author of the video.
3. Don't be afraid of making a fool of yourself. If you guess wrong, no harm done.
4. If you know more about the subject, don't be afraid to share.
5. If the author responds but seems confused, follow up with another comment.
6. If someone else flames you tell them to fuck off.
7. Don't flatter anyone who posts its a waste of time to those reading a wall of comments, too sentimental (and boring).
8. If you like to see more just say so.
9. Read as many of the comments that you can before posting so your not repeating someone else.
10. Remember that the poster does not own his youtube space, its a creative commons, rude comments are allowed.
nico