I've had it with people using the "embed youtube video into post" function the wrong way. Everywhere I look: videos that won't play and quite frankly, I can't blame you. It isn't self-explanatory. However I can't fix this in each post and I won't explain it to each new user, so I will explain it here once and for all. 
Imagine you want to post a video of a kitten riding a turtle. The URL is this:
In order to embed such a YouTube video into your post, what you would most likely do is this (without the * of course):
[*youtube]
[*/youtube] What you end up with is this:
http://www.youtube.com/watch?v=
The problem: you put the entire URL between the brackets you silly bear! Try to only put the Video ID between the brackets, it's the code after the "?=" in the URL and in this example:
[*youtube]JS-QX061KHU[*/youtube]
Without the * that I used for demonstration purposes the video will display correctly:
Success! 
Happy posting!