This right here is interesting. I think people get confused with the illusion of game design.
For example I'll use what Mr.Mumbles said. "What would Riachu bring to the table that Pikachu doesn't?" In Melee we have seen Pichu pretty much replicate Pikachu with some minor difficulties and a different model. Does this make Pichu stand out as a character due to his model or due to his attacks? If you look at the semantics, there probably are just integer changes between the characters (by this I mean changing speed, damage, etc). Now what if we gave similar move sets to other characters and leave their model the same? For example, let's make Daisy a Peach replica. Does this mean that Daisy should be worth a spot in the roster?
This is what everyone should take into consideration when determining the abilities of characters. You can visibly make a move look different (Mario's FSmash vs Pikachu's FSmash), but it needs to define the character. Although, Mario's FSmash and Pikachu's FSmash are almost the same if you look outside the visual concepts. The things that are different are very minor changes to Mario's FSmash (such as damage, range, etc).
Its way more interesting to have a WAY different attack such as Megaman's FSmash compared to anyone else's FSmash in the game. It makes this character play WAY differently from the rest. This is what makes a good character. I hope to see more interesting changes with Smash 4.