I did an experiment where I uploaded the same video over and over again trying different things. I found a four word tag which only had 15,000 results, so it was easy to rank in. Every time, I deleted the video, and re-uploaded again with different variations. This was all done in incognito mode.
I uploaded the video with the same 4 word title, description, tag, and ranked first on the exact search, and 2-45 on a few 3 word combinations of the 4 word tag. Removing the description, or tags separately made no difference at all in the ranking. Removing both the description and tags, I still ranked 1 on exact search, but on other searches I dropped by 1 rank. So, title is by far the most important. I tried a compound tag, and separate words, made no difference in ranking. In one test, I kept the title the same, but made an irrelevant description, with random, but unique tags. None of the words were from the title. I ranked the same on all the tested search terms, but on one I got a rank lower.
Then I changed the title to a different 4 word tag, which was also easy to rank in. The description and tags were the same as before. For the new tag, I ranked easily on many 3 word combinations. For the old terms however, I dropped a lot in the rankings. I then repeated it without the tags, but kept the description, it had zero effect. Next I put back the tags, but deleted the description, and now I couldn't find my video at all for the old search terms. So new title, old tags and description, made a big drop in rankings for the old keywords, and once the description was removed, I couldn't find the video at all.
Next I repeated the experiment keeping the old 4 word title, but this time I made a long drawn out description, mentioning each of the keywords several times, For tags I used a combination of long tail tags, and single words, and even irrelevant words. It made no effect from when I just had the 4 word description and tag, in one of the searches I improved by 3 spots. Then I changed the title to the new one from the previous test and repeated it. This time I dropped by about 10 spots in the ranking. So if you spam tags that are in the title/description, it seems to be ok, but if you are spamming tags and description that aren't part of the title, I think it will negatively affect you.
Finally, I found a four letter tag which only had 10 search results. I used it as my tag. No matter what I did, I could not show up in the search results for this tag as long as it was left out of the title and/or description. So if you put PewDiePie in your tags, but it isn't part of the title or description, you might think you are one of the millions of results when people search PewDiePie, but you probably aren't.