Well, the two I expected to main are Bowser Jr and Ness, and those are the two I've been successfully maining online. However there are characters I expected to like more than I actually do.
Like Mega Man. I'm not sure what I was expecting, his moveset makes total sense, but his biggest kill options seem to be the uppercut (which requires you to hump your opponent before you use it, which is risky because you could get caught... with a lawsuit...), the side smash (which allows Mega Man to glow with a huge "KILL ME" sign just before he uses it against any character who can reflect), and throwing your opponent off the stage followed by trying to spike them (which is what I usually have to do).
Robin is overall about as good as I expected him to be, though not in the ways I expected. He is slow, as others have pointed out, but he's got some pretty good smash attacks. I don't think he's bad, not at all, I just didn't expect him to play the way he does. I dunno. I thought he'd be a bit quicker (so he could actually stay 3 steps ahead) and I thought his spells would be a little stronger.
I guess Dark Pit would count as a character I was disappointed with too, cause I went from hoping that MAYBE he was an original character, to hearing stories that he was a clone of Pit from Brawl, to finding out that he might be the cloniest clone of all smash 3ds clones (I'm pretty sure that canonically he's an actual clone, right?). Still I hoped he would be something else, and I hoped that Pit would have more kill potential in this game since he felt weak in that department in Brawl.
I believe that everyone else in the game either feels as good as I expected them to or better (except maybe Pikachu, he feels a little weaker than I remember).