[SahAssar]

There are differing opinions on this from what I've heard. If the let's play video competes with the experience of playing the game then it seems like it would be as you say, but for many cases that would probably not be true. This is still not decided in court though and just because someone can DMCA someone else does not mean they are in the right legally. It would also be different in different countries, the free use doctrine and DMCA are a American concepts.