I personally think 19 characters is a pretty well rounded cast. It's only 2 less then Mario Hoops 3-on-3 (Most of the missing characters that were in Hoops were part of a generic species. I know Yoshi is technially one of those too but let's face it he's alot more major then Birdo, Boo, Fly Guy, and Paratroopa). Sure, Rosalina and others would have been nice but just because they're not in the game doesn't mean it ruins it. If you really dislike the fact that Slime and FF characters are in this you still have 13 Mario characters to use
Like I said before the factor that will sell this game in the long run will be the Online Play. Without that if you buy it, it would just go in the same shelf as you do with other Mario sports games until you feel the erge to play one of them again with a friend over at your house.
Also you can't blame this on Nintendo really if your that mad about the roster. If you have to point a finger at someone, do it at Square Enix considering they're the ones who developed it.
I'm pretty sure it's only a matter of time before people stop complaining about how many characters there are to a FC sharing thread for this game in the Online section.