Ultimately, keywords (and keywords aren't just tags...keywords should be reinforced in all SEO metadata, which includes titles, descriptions, tags, and submitted subtitles) are about describing your videos accurately to the youtube algorithm so it can make associations about where your content should be shown to what audiences in search results or suggested videos. The basic tension is that shorter tags usually have more traffic, but shorter tags are also usually over-competitive, meaning that there are too many videos going for the same keyword for you to be able to realistically rank well for those terms. Longer tags may avoid competition, but the risk is that they may be too focused, and not as many people may be searching for those terms. (Ranking 1st for a term that NO ONE searches for will not help you at all, for example.)
There are a few different types of tags that you should be using for your videos, I think. Broadly, I'd say that the two types of tags there are are tags for search and tags for association or branding...but I'd probably divide them out even further, like so:
1) Long tail tags that people would be searching for that you could realistically rank for in search
Let's say you're doing a video on Battlefield 4. Just using "Battlefield 4" as a tag is too generic and too competitive...there's little chance you'll rank in search. So you have to think: what specifically is the video about? And then generate tags (and descriptions and titles) around that. For example if your video is more specifically about "battlefield 4 c4 trolling" then you'd want that entire phrase as a tag. (however, you have to ask yourself: how many people are searching for that phrase? There's probably fewer people searching specifically for "battlefield 4 c4 trolling" than "battlefield 4", but as long as there are a decent number of people searching for that term, and you have a good shot of ranking in the 1st page of search results for that term, then it's a worthwhile tag. In this case, when I type in the youtube search bar, "battlefield 4 c4" youtube pops up "battlefield 4 c4 trolling" as an autocomplete suggestion...so that suggests to me that at least SOME people are, in fact, searching for that term.)
I'd say most of your tags should be longer tail tags like this. You should think about what sorts of specific things people might be searching for. This is an art and a science -- keyword research takes a while to get used to (and you may find that you have to start adjusting your content to anticipate what people are searching for.)
EDIT: Keep in mind that tags don't add. So "battlefield 4" "c4" "trolling" assumes that you are targeting people searching for those individual words (which is not what you want, generally). "Battlefield 4 c4 trolling" assumes you're targeting that collection of words.
2) Shorter tail tags that people would be searching for that you probably wouldn't rank in search, but which are good for association
OK, so a tag like "battlefield 4" is probably too competitive. You're competing with EVERY battlefield 4 video (whereas when you get more specific, you can narrow your competition). Does this mean that short tags are worthless? Not necessarily. By including these short tail tags, you build associations...so youtube may put your video as a suggested video for other videos about battlefield 4. So, I'd say if you have "battlefield 4 gameplay" you'd probably want that as a tag, although you should be aware that there's so much battlefield 4 gameplay that you're likely never going to rank for that kind of term. This is the challenge for gamers -- there are so many gaming videos that youtube could (and does) fill the first several pages of search results with established YouTubers' videos.
3) Tags that you don't think people would be searching for at all, but that build brand associations
If you tag your channel name, for example, then you shouldn't expect people to actually be searching for your channel name, but what this does is it causes youtube to associate your videos to each other, so that youtube is more likely to recommend your own videos as suggested videos when other videos you've done are playing.
