Go to metacritic and compare the Wii's library to the 360's and PS3's. It becomes quickly apparent that the Wii has a growing list of subpar games and relatively few good titles(above 80-85). It's true that SMG, SSBB, and MP3 are coming and all should review very well but it also is true that what appears to be a tsunami of crap will also wash in with it.
I think Nintendo is partly to blame since they continually tried to push Wii development by mentioning low cost for making a game on the Wii (sub 5 million and when you factor in the advertising costs into that it leaves very little for developing a quality AAA or even A game).
Also the fact that Nintendo is publishing poor games themselves (Mario Party 8) which are having huge sales doesn't help the problem.







