Yes, you are completely right about that - of course it does not apply to any type of game, however I would say it could be the case for any game that has some kind of story or plot or levels embedded. There are quite a few titles, that I did not buy thanks to those videos: ranging from Indie-title like LIMBO to AAA-titles like Diablo III - I think Lets Play Videos are great! - Of course you cannot replace a good session of multiplayer-fun with friends(or strangers) - but I still believe that they might potentially harm (sectors of) the gaming industrie, even though I like them a lot!

Heads up for you website! (just as a feedback: why the mandatory signup/registration?)