My thoughts on the leaked characters:
Shulk: YES!! I've expected him ever since the second part of the Gematsu leak surfaced, and after playing Xenoblade Chronicles he's become one of my most anticipated potential newcomers! We've seen a bit of his gameplay but it's still hard to tell exactly how he'll play, but his Monado Arts mechanic seems really fun and interesting. I wonder just how many of the Monado's abilities will be represented in his move set, and in what way. I'm also curious about his alternate costumes, which I hope includes his armor (or lack of armor ;D) from his game.
Bowser Jr.: I didn't expect Bowser Jr. at all, and he wasn't a character that I particularly wanted, nor did I think he deserved to be in Smash Bros. But I also didn't expect him to be in his Junior Clown Car if he did get in! It makes him a very interesting and unique character, and the addition of the Koopalings as alternate models for him essentially makes him an 8-in-1 character! I'm really excited to try him and the Koopalings out. Sure, it's ANOTHER Mario character, but it's another villain, too (or 8, if you count them separately, lol), and we need more of those!
Duck Hunt: As crazy of a choice as it seems, Duck Hunt Dog was actually a fairly common suggestion since the pre-Brawl days, so the idea of this character doesn't necessarily surprise me, but I never expected it to actually happen! In hindsight, it's a pretty darn good choice for a retro character, considering Duck Hunt is technically one of the best selling video games of all time... Of course, what intrigues me most is just how this character will play. The fact that the character is named "Duck Hunt" and not just "Duck Hunt Dog" or "Dog & Ducks" leads me to believe that this may be a three-part character: The dog, which is the main character that you directly control on stage; the ducks, which assist in moves (perhaps not cooperatively, but instead by provoking the dog in a game of cat and mouseor dog and duck, rather); and the off-screen hunter, shooting into the screen as if it were really a game of Duck Hunt. Whatever the case, this will surely be a very interesting character.
Dark Pit: Again, another character that I didn't expect. Of course, prior to Lucina's reveal, I didn't think that this game would have clones, so I didn't even consider Dark Pit as a possibility. But I absolutely LOVE Kid Icarus: Uprising, and I love Dark Pit, too, so I'm definitely not complaining! And even if he is somewhat of a clone, I imagine he'll be different enough from Pit, considering that he has the Dark Pit Staff, not the Palutena Bow (or any bow for that matter) like Pit has, so at the very least some of his moves have to be different. I would think that Sakurai would give him some other weapons from Uprising for some of his special moves, too, possibly different weapon classes than the ones Pit uses.
Dr. Mario: Not a newcomer, of course, but definitely an unexpected returning veteran! I wasn't really a fan of Dr. Mario being his own character in Melee, and I didn't really want him to come back unless he was just an alternate costume for Mario, but I realize that in that case he would loose his unique properties like his Megavitamins...so I guess I kind of understand why Sakurai would decide to make him his own character again. I'm guessing he and Dark Pit are in the same boat as Lucina: Originally planned as an alternate costume, but ultimately fleshed out a bit more and given a separate slot due to slight differences in gameplay. And in that case, it would make characters like Dr. Mario, Lucina, and Dark Pit essentially "bonus" characters, and I can't really complain about that.
Of course, there are also the characters we HAVEN'T seen yet... While we don't know if there really are more characters than what we've seen in the leaked roster or not, I'd like to believe that there are a few more. Ideally, I'd hope that everyone from Brawl (except Squirtle and Ivysaur, since Pokémon Trainer is no more...) returns, plus Mewtwo, too, and there are a few additional newcomers that I'd really like to see as well... Here are my thoughts on some of them:
Dixie Kong / King K. Rool: With Kid Icarus having three characters, Fire Emblem having four, and Mario having a whopping SEVEN (and that's not including Yoshi and Wario), the Donkey Kong franchise feels lacking in Smash Bros. I think a third Donkey Kong character is overdue, and either Dixie Kong or King K. Rool would suffice. If Star Fox and Kirby got three characters, why shouldn't Donkey Kong? Dixie Kong would be unique by using her hair to attack, even if much of her move set ended up being based on Diddy's, plus she'd be adding another female to the roster, which is always welcome. King K. Rool has plenty of potential for a move set based on his various boss battles, and he'd make a great new villain character, of which we have too few of in Smash Bros.
Chorus Kids / Marshal: I thought that a Rhythm Heaven character being in Smash Bros. was merely a pipe dream, but as we've seen, quite a few leaks have mentioned the Chorus Kids or, to a lesser extent, Marshal. Either one would be fantastic, and I can imagine them representing the Rhythm Heaven series as a whole though their move set much like Mr. Game & Watch does with the various Game & Watch games. I really, really hope it happens. We've already seen a Rhythm Heaven enemy in Smash Run, so it may be a safe bet; however, we've also seen the Polar Bear from Ice Climber in Smash Run, and yet the fate of Ice Climbers in Smash Bros. for 3DS/Wii U is still up in the air. Speaking of which...
Ice Climbers: I understand Squirtle and Ivysaur being cut since transforming/switching charactersand with that, Pokémon Traineris no more, and I would understand Snake's absence as well, given his third-party status. Even Lucas and Wolf being cut is somewhat understandable, being semi-clones that only have one Smash Bros. game under their belts, though the loss would be painful. But Ice Climbers, a starter character since Melee... Them being cut sounds insane. Of course, if it does happen, I'd imagine it's due to limitations of the 3DS version. However, if we were faced with two optionsto have Ice Climbers be cut completely, or to limit the character to a single, solo Ice ClimberI would happily accept the latter. I actually liked playing as a solo Ice Climber in Smash Bros. when I lost Nana, and it just felt more...right, I guess, than commanding two Ice Climbers at once. After all, you never control both Ice Climbers at once in the original game. I'd be perfectly content with a buffed solo Ice Climber with a new Up Special (the Condor, obviously) if it came to that, and there's even one rumor out there that claims that may indeed be happening (though there's no real credibility to it, sadly). While it would mean a unique mechanic is lost (wouldn't be the first...*coughPokémonTrainercough*), at least the general move set would still be kept, and we'd still have our sole ice-based character. I'd certainly take that over the character being cut completely.
And, finally...
Mewtwo: Because DUH. It makes SO much sense for Mewtwo to return, because it's more relevant than ever now, having starred in a new movie and being featured in the latest Pokémon gameswith not one, but TWO new Mega Evolutions at that! And since Generation V it's even gained a new signature move in Psystrike! Not to mention it's by far the most wanted cut Melee veteran character. Mewtwo has so much going for it right now, and I'll be really disappointed if Dr. Mario, another Melee veteran, returns after being cut in Brawl but not Mewtwo. If Mario can have SEVEN characters, then I think there's no problem with Pokémon having a sixth (especially considering that there were already six Pokémon characters in Brawl if you counted Pokémon Trainer's Pokémon separately).