Surprise to me that a Chinese phase manages to slip into this list of popular tags, 高音質, 高=high, 音=audio, 質=quality, and the list is not a mess of everything, makes sense as it has a much smaller audience
Apart from the tag generating function, the other function is to look up the tags used in existing videos. With this I figured why I had hard time finding certain maikel's videos - the tags are not optimized. TL;DR at bottom.
eg Nightcore - Angel of Darkness, Maikel6311
The song name 'Angel of Darkness' are separated into 3 tags, instead of sticking together as a single tag. IMO the most important tag, 'nightcore angel of darkness' <-all in one, is missing too. The result of the absents of these crucial tags are obvious, the video ranks below 4 other videos in search result of nightcore angel of darkness on YouTube, despite having much more views than the above two videos.
So what makes the above videos rank higher?
Tags, duh!
Angel of Darkness- Nightcore 10 hours, Tralfazz74
tags
blonddummikaddi, request, lyrics, 10 hours, tenshi, angel of darkness, nightcore
Despite having much less views, only one year old and less like ratio, it ranks higher. All for a simple 'angel of darkness' tag.
So the trick is, if the song is more than one word, tag the whole song name as one tag not separate tags. And add a tag of the nightcore plus the whole song name 'nightcore song name'.
Welcome to Nightcore Universe! Enjoy your stay as much as you enjoy the original Nightcore songs!
Useful threads/links
Nightcore - That Night - YouTube
Aug 3, 2011 - Uploaded by Maikel6311
Maikel has just recently used a Python script to overhaul the tags, and this is the video I asked Maikel to improve the taggings. Before the overhaul, the tags are like
nightcore, that, night, each, word, is, a, single, tag
I guess it was how that tags worked in the old YouTube but now it's possible to have a number of words in a single tag and it made single word tags obsolete, e.g. video with single word tags ranked lower (at least from the exprience of searching for Maikel's videos). Now the Tags are
Nightcore - That Night, Nightcore, Nightcore songs, Nightcore (Musical Group), Eskimo & Icebird feat. Maria Merete - That Night, That Night, Maikel6311, Maikel631, Eskimo & Icebird feat. Maria Merete, Eskimo, Icebird, Maria Merete, Nightcore Maikel6311, Maikel6311 Nightcore, Nightcore Mix, Nightcore Ultimate Mix, Nightcore 1 Hour Mix, Nightcore Gaming Mix, Nightcore 2016, Nightcore 2015, Nightcore 2016 Mix, Nightcore 2015 Mix, Nightcore - God is a Girl,
And it did brought the video from the middle of the second page (2 years ago) to the middle of the first page. Maybe it's YouTube changed it's ranking system or it's the new tags worked.
Nightcore - Fallin' - YouTube
Jan 20, 2011 - Uploaded by Maikel6311
It's mostly about quantity, consistency and relevance.
More tags equals more possible search matches.
If your videos all share a common tag, then you are more likely to be prioritized over someone using that tag for the first time.
The more relevant your tags are to the search, the more likely it is to be ranked higher. That also works the other way too, if you have tags that don't relate to a search it will be ranked lower.
Another important factor is tag order. The more specific your tag is (song title, artist name), the closer it should be to the beginning of the list since the first tags have a higher weighting.
Also, single words like 'and' or 'that' have no effect since they are too generic. That is why you shouldn't split up tags unless you have to.
Now this is speculation, but I don't think repeating the same tag with the words in a different order matters, since from my experience it doesn't actually matter in which order you place your words when searching, you get the same results with just tiny differences, unless you match a tag or title/description exactly.
Though, of course the biggest contributor to your search ranking is channel and video popularity, not tags at all ^^