Well let's say you go with the example I gave above: a Black Ops 2 gameplay video with a post commentary about difficult exams at school. You could use tags like:
Difficult exams, GCSE, A Level, Finals, Exam paper, black ops 2, microsoft, gameplay, commentary, post commentary, let's play
I could go on but these are just some examples. Think about what people might type into a search engine. If you are doing a post commentary about passing exams, type in 'how to pass exams' as a tag. It all helps, and it's all legit so you get the right audience