Well, like he said in his video for the first BioShock, "no one likes it when I'm nice to a game", and he elaborated in an Extra Punctuation column that any aspect of the game he doesn't trash in a video should be assumed to be fine.
Yahtzee is banging the "games are art" drum, and it's no wonder that would be a disappointing position to take. Most games aren't art. Quite a large number are uninspired, derivative trash [http://en.wikipedia.org/wiki/Sturgeon%27s_Law#.E2.80.9CNinety_percent_of_everything_is_crud.E2.80.9D]. That doesn't mean they can't be fun to play.
Every other medium is also full of trash, some of which is absolute dreck and some of which is still entertaining. I would never hold up G.I. Joe: The Rise of Cobra to the same evaluation as The Godfather or The Seven Samurai, and it will probably be all but forgotten in a few years, but it sure was a fun way to spend 2 hours. So yeah, Yahtzee's correct that most of these games are derivative, have crappy writing and poorly-designed characters. But they can still be fun to play, even if they won't be remembered a few months down the road.
?Criticism may not be agreeable, but it is necessary. It fulfills the same function as pain in the human body. It calls attention to an unhealthy state of things.? -Winston Churchill