Marth's and Ike's presence are givens. They're cool characters.
I really like Robin's inclusion. Being able to play both versions of the player character from FE:A is pretty neat. Plus it's really cool to have a non-swordsman Fire Eblem character in Smash Bros, so I like his/her arrival into the scene.
From what I understand, Lucina is a *really* popular character in Japan. Granted, I've only gleaned this from hearsay and rumor so take it for what it is. Sudden realization: Nintendo is including Lucina so that everyone can play with their waifu . If Nintendo thinks it makes enough sense to include Lucina for the Japanese market, it wouldn't make sense to NOT include her in the US version. All the animation/art would already be done. The only thing to change would be the language.
As someone who hasn't played FE:A, Lucina seems like a repetition of Marth. Not only does she look exactly like him, the move set is identical aside from her animations and the sword-tip mechanics. Marth's sword does more damage at the tip while Lucina's is evenly spread. I'm really torn here because I like her character design, but I've mained Marth for years and the small differences that we know about Lucina's kit tell me that Marth will be more engaging to play. She feels like another Marth. We already have a Marth! Why do we need another Marth?!? I really love the idea of a badass swordswoman, but her kit just doesn't seem unique enough from Marth to compel me to play her. I, however, reserve my final opinion until after I play her when the game comes out.
But the real question I think we ought to ponder is whether or not Marth will speak Japanese in SSB4 now that he has FE games featuring him in the US. I earnestly hope so, but I doubt he will. Taunting just won't be the same in English.