As you may have noticed, the embeds from YouTube currently (in the beginning they did not) allow the display of annotations. So apart from the placement of introductory leaders (e.g. videos from ONN) or a logo/URL in a corner (e.g. viral sites who take content from others without attribution) there are now more ways to add subjective opinions / advertising to a video.

I'm against the addition of subjective opinions / advertising as I think they interfere with the viewing experience.
The only 2 exceptions for me:
1. when - at the moment of posting - there's no other alternative video available without these unwanted extras: then it's better to have the video, despite the ugly additions.
2. constructive use of (YouTube) annotations in case of subtitles (also misheard ones, e.g. this sift).

But e.g. this annotation ("is ment to be played lowd" (in the beginning)) I think is avoidable.

So what do you think should be allowed and what not?