Latest tagging info from a YT guy, I believe partner manager (warning: translated and paraphrased

)
- Choose phrases that viewers will actually type into search
- If the phrase is too specific to be searched ("sunrise in cologne"), split it ("sunrise", "cologne")
- More tags means less weight on each tag
- But too few tags means missed opportunities for people to find the video
- Use about 20 tags, 10 of them relating to the video, 10 of them relating to the channel (used in every video's tags)
According to Tim Schmoyer (YT certified), half of the video-related tags should be specific and the other half generic.