I think to determine weather he likes a game or not is abit more esoteric than it seems.
He knows when he's making valid, reasonable critism and when he's nitpicking, and if he's nitpicking, it can imply it's a flaw in a game you're likely to overlook which he doesn't expect you to take seriously, and if he's not, then he's telling you about flaws in a game you should be aware of before deciding buying them, and the trick is determining weather or not he's nitpicking.
Essentially, that's sort of why whenever I play a game he reviews, I seem to take it from a similar perspective to his own, if he thinks a game is good or bad, I seem to look back at watching whatever review he's made and make a shocking (or maybe not-so shocking) blow of finding him to be totally, totally right about a game, the only exceptions I found to this were Fable 2 and Soul Calibur 4, both games I played and thought were good.
Being the sort of gamer with no job and rarely getting money, I need to be very careful with what games I want to get as it would be a long while before I can get another game. So I came to rely on Yahtzee's reviews before deciding weather to get a game or not for the reasons above, and he has helped discourage me from getting certain games that I either had my eyes on, or never heard of, such as Demon's Souls, or Haze.