Hmm...If I had to point out a few of the worst offenders (in my opinion of course) than I would definitely choose Palutena, Mii Fighters, Little Mac, and Shulk.
As a disclaimer, I openly admit that I have not played these characters at all. This is all analysis, and is open to criticism within reason.
Palutena:
To start with Palutena, she was a character revealed to us as the "jack-of-all-trades" kinda gal, and that was cool. She had a move for just about every situation and they all completely differed in functionality. She was literally the official (by that I mean non "create a character" style such as with Mii Fighters) character that could do it all. That is until it was revealed that ALL custom moves were banned from online play. Meaning that a good 8 of her possible moves, each with a different functionality and each being a possible crucial asset to a player whom might main Palutena in the future, just went flying out the window. They kinda just gave her a cool gimmick that probably took a lot of hours to work on, and then they just threw it away. Sad.
Fighters:
Mii Fighters suffer from this same problem. Nice gimmick, probably took a lot of hours to implement, got many players hyped, then got the boot from the main mode that mattered, but wait there's more. They also unfortunately suffer from what I like to call, "Doing way too much" syndrome. What I mean is they have too many option for customization that they may never see official tournament play because of their weight, height, power attribute thing. I personally find it weird that they were advertise as "anyone can join the fight" but not with "anyone" if you know what I mean.
Mac:
And then comes Little Mac, the scourge of the grounded plains! Enemy to all land-walkers! Master of the fist! ...why is this guy SO bad in the air? I understand the idea they were working with, and in all honesty it was a good one. We have Jigglypuff whom is queen of the sky, but not much of a threat on the land. However unlike Mac, the degree of her limitation do not contradict with the format of the game. Smash Bros in general is a platform combat game. Platform meaning, your...behind will be seeing a lot of airtime if you play this game, and as much as I like Mac's design, I think they went a little too overboard with his limits. I wish I could point out one specific problem he has when in the air but I can't because all his options in the air are completely horrid. His aerials are 'meh', his recovery is a joke, and anti-air ain't the greatest. On the flip side we have Puff whom can "wall-of-pain" off the stage and camp in the air, but can also rely on her powerful smashes and decent tilt (mostly up-tilt) when shes on land. To be completely honest, I hope Nintendo releases a patch that gives the man at least a decent recovery (he can keep his aerials the way they are but the man needs to be able to recover!). There isn't too much worse than seeing someone lose because they got thrown off the stage at 20% and died because...Little Mac.
Shulk:
Now Shulk is a mixed basket. I haven't seen much of him but what I have seen of him has left me unimpressed. I feel that his Monado Stance ability is a very good gimmick for a character, especially a swords man, to have. Unfortunately, just as with Mac, I feel they gave him something really good but took something more important away. That something being flow, and Shulk just doesn't look like he has much. Now this is all an opinion from videos of other players but from what I've seen it seems as though Shulk was built with too much balance in mind. To explain, I feel as though the dev team designed Shulk as a below average character in hopes that his Monado Stance would give him the edge he needed to be above average, and that does not seem like the case. He looks much to sluggish for a character of his weight class (weight of captain falcon but moves like Ike with way less power), his natural speed is okay, his air-game is decent and his Monado just doesn't do as much as what may have been hoped for. Not to say that the Monados do nothing, they do help but the fact that they are one at a time use with a time limit and they each come with a counter effect just don't help balance his flaws. To put it in better perspective, take Kirby when he absorbs Shulk and gains the Monados Stances. Yeah they help boost Kirby's power for a good time and make you feel like you really are getting a benefit with a fair drawback, but when the effects go away you still feel like you are playing a solid character not a character that NEEDS that ability to be solid. In other words he shouldn't have to rely on his gimmick to be a decent character. He should be a decent character that has a gimmick that makes him good...for a limited time only.
As a side note I also think "Back-Slash" would be better as forward Smash. It could keep the added effect when hitting a player from behind and it would free his side-B for something that gives him a little....more.
TLDR: Reading is important for the mind.