Look this is the way I see it. Take a party game like Smash Brothers Melee, which is virtually perfect at everything it does. Like the game or not, it's a well made game with very few flaws, it's has a ton of depth and excels in every area. This is a 9/10 caliber game.
Take a game like Mario Party, another party game that doesn't score as high, the game has a TON of flaws that have been apparent since the N64 versions that have never been changed, the controls for the Wii version don't even work as advertised and feel really tacked on, the game lacks content compared to other MP games in the series, the consistent problem of playing against the AI is still present in this version, the mini games repeat themselves very often. I'm a big Mario Party fan and I loved the original 2 but the Wii version has some very apparent flaws and this game surely isn't 9-10 material.
Same goes for Wii Play, the reason it sells so much is because it comes with a Wii controller. I've never heard anyone say a positive thing about Wii Play aside from praising 1 of the 9 games. The game has no depth, the controls are poor for some of the mini-games, and often times there is no creativity in the games at all. This game, fun or not, does not deserve an 8-10 either.
Critical theory would support what I'm saying in this case. There are plenty of party games that score well, most just aren't on the Wii so there MUST be some kind of problem with reviewers.
Now someone tell me why games like Mario Party, Wii Play, Big Brain Academy or Wii Sports deserves a 10 without bringing their opinion into play. That means you can't just say 'it's so much fun it's a 10'. You have to prove that these are extremely well made games that are the pinnacle of the genre, do almost everything correctly, are full of content and offer great gameplay. Have fun trying to argue that Mario Party and Wii Play deserve the same scores as something like Super Smash Brothers Brawl.