While I fully agree that Wii Sports has been remarkably influential, most of the people arguing for it in one way or another seem to be skirting one major issue, which is that unlike, say, super mario, it's an extremely uneven game.
I have also seen my parents (who are in their 60s) play the first video game they've ever tried, Wii Tennis, and look like they're having fun. I've played Wii bowling with neighbors who never owned a video game system before the Wii. However, I've also turned on another game and laughed at how bad it is, along with a room full of people who've almost never played a video game before between them, but who can still tell that whatever Wii boxing is trying to be, it's not a success.
I think the confusion in the discussion comes from the fact that video games are an art or craft (take your pick) that depends upon technology.
Obviously any serious argument about Wii Sports being an extremely influential game will center around the controls, which were only possible due to a technological innovation in the form of the Wiimote.
As an analogy, the first recorded music was something that would change the way humanity thinks about music, one of its most elemental creations, forever, but the inventors of recorded sound probably shouldn't be seen as a musical geniuses on the level of Beethoven -- and neither should the first people to release popular recordings. This is the argument that says Wii Sports doesn't have to be a great game, and can still be important and influential as a jump forward in technology.
The confusion comes in because video games are inherently a technological medium. Many innovative games involve either software or hardware advances in technology. So, while it often seems we can isolate the kind of craft essential to dreaming up games like Braid, and find a kind of creativity not dependent on technology within game development, things become harder to define when we're talking about something like Wii Sports, which involves strikingly new hardware. When some people talk about a great game, they're trying to isolate just the genius that creates a game like Braid -- while others see what is obviously a brilliant invention and can't believe others won't acknowledge the equally obvious significance of it.
I don't know that there is a right answer to the question.
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