The main idea behind copyrighted materials is prevent others from gaining money directly with your product.
When it comes to media, in copyright laws, there are clear fair uses of what's allowed and what not.
It's not even allowed to post pictures of a movie on the internet without asking the copyright holder for commercial purposes (otherwise, it's just "fair use" - unless you post enough pictures that it tells the story of the movie).
Commercial means that you get money by using the content.
For example, if you post the description of a DVD cover and some pictures and link to amazon to buy it (with getting profit from that), that's clearly covered under copyright.
Writing a critics of the movie using your own words and posting pictures with that is a citation, and that's allowed (even when you make profit with it).
Now, with video games, it's the same:
Showing part of the game, reviewing it, or explaining how to pull of certain tricks, etc., is okay. It's your own content which just does citations of the game.
Doing a full playthrough of the game, even if you explain what's happening, is NO derived work, because that's part of the experience: Playing the game and experiencing the story.
Even if it's just a bit of gameplay you show (I.e. one minute playing Zelda):
If you just play without any own work, it's covered under copyright laws.
But even for citations there are limits: In order to count as your work, the majority of the video has to be your work: So when someone plays a game while babbling something along, it has to be checked individually whether this is new works or not.
In most case it isn't - as the main content in playthroughs still is the game.
It would have to be a good review, but a review doesn't need to show the full game.
So it's like that:
* Playing a game without any added content (regardless of the length): Okay when you're not making money (so: no ads, etc.). Otherwise: Copyrighted.
* Playing a game through while commenting what you do: still the same
* Doing something like a review of a game, where the main part of the video is your review and not any content of the game: Okay, even for commercial use.
* A game being played in your movie in a small scene: Okay, as the content of the game is not important, it could be any game that's being played there.
Most videos in the internet fall into the first two categories, so Nintendo can actually do something about them.
Especially if longplays of games DO damage. A big part of a lot of games these days is the story - and quite a few people are happy just watching someone play the game, so they might've bought it but didn't have to anymore.
Note: That's all according to copyright laws in Germany, but it's probably very similar for most civilized countries.