Fair Use doesn't have strict boundaries. All of the above factors are taken into consideration when a judgment call is made. If it's for education/non-profit/etc., then that's a point in favor of it being fair use. If it's commercial, that's a point against.
But the biggest, most relevant thing here is the amount of the copyright work being used. There's no way any court in the U.S. is going to say a "Let's Play" video is fair use in a commercial setting. Even tiny musical samples are often ruled not fair use. It's not even close.
So, let me ask you a question, then?
When you play a game, how do you play it? You play it in your own way, do the things that you want to do in the game, you pace yourself, go to a certain spot in the game at a certain time, kill a mob a certain way every time, whatever, right?
Take another person playing the exact same game, then. That person plays it differently. They don't go to that very exact spot at the same time, they don't kill the mob with the same weapon, they try to get through various spots as fast as they can, and they usually don't grind out things as much as you do, right?
Now, you BOTH record what you do, and you both post something to YouTube! The playing of the game is then your own because you played it your own way. The creator created the world for you to play the game in, but YOU chose how to play that very game, and it shows when you compare the two playthroughs side by side!
And therein lies the gray area! Because the content creator did not, in turn, create that playthrough and did not tell you how to explicitly play the game and how to pace or whatever, you should have some right to not only show that particular playthrough of the game because you took the time to play that game for your audience, but to be compensated in some way because it's YOUR playthrough! It can be their world, but your choosing how to play the game and where to go at whatever times.
The problem is that we're treating games the same way as movies or books, which is not the same in any way because they are not static viewings. You cannot control how a movie plays out, or a book, but you CAN control actions in a game because it's an interactive piece of art, which basically means that the rules will become much more unclear as to what should or shouldn't be considered yours in that playthrough. Why do you think there's more of an outcry over the used games industry as opposed to used good industries in other forms of media? Because there's that whole static versus interactive thing that you need to consider, and though I support the used game industry in every sense of the word, the thing about it is that games not having a static way to play makes a TON of difference in this debate.