When I try to use the "spoiler" format on a bare link, the result is just an exclamation point and the one boxed link. See example here. Many YouTube videos contain spoilers in their title, as was the case here. I realize I could put the link in text and force it out of one box (and I may do that in this case, pending the response to this question). However, "one box" is a very attractive feature to question-askers, so I'm torn about which feature to use. Especially when it comes to YouTube I'm much more inclined to visit the video when I can see the preview.
I support this feature-request for images, but I don't think having it on youtube will be a good idea.
Spoilers are revealed when the mouse hovers over them. Once you start playing the video, having it hidden or revealed based on where your mouse is is not a behavior I find reasonable, while on the other hand having it behave any other way is inconsistent with the rest of the spoiler box.