Here's my tier list in its entirety (I'm making such a big deal about it here because I'll probably make a video version of it eventually).
A+:
A:
A-:
B+:
B:
B-:
C+:
C:
C-:
D+:
D:
D-:
Instead of putting out character specific explanations most of which I've already done in two previous posts, I'll do something different and make categories that explain some crucial strengths and weaknesses of different characters. I'm not going to list everyone, just high and low tiers to justify why they're the way they are. Mid tiers are generally mid because they don't possess particularly polarizing traits of any sort, or because they possess both positive and negative ones. Basically the entirety of B- to D "has some really cool stuff" but as we go lower their negative points get more and more noticeable. For example, Robin probably has the best aerials in the game overall, good projectiles and decent CQC options, but is very slow on the ground and struggles to reset to neutral. Go lower along the mid tier and we have Samus, she has a good projectile as well but her aerials are "only very good", and she struggles to reset to neutral even harder due to her floatiness. Her CQC also is somewhat worse mainly due to her slow grab even though she has up b OoS for instance. Next, go even lower and look at Mewtwo or Mac and now you should see the trend.
Godlike frame data / good frame data: Sheik, Ryu, Mario, Diddy, Fox, Pikachu, MK, Luigi / Rosalina, Ness, Pit, Dark Pit, Peach, Mega Man
Great mobility / good mobility: Sheik, ZSS, Sonic, Fox, Pikachu, Falcon, Yoshi, Greninja / Ryu, Mario, Diddy, MK
Campers prevalent enough to control the match: Sheik, Rosalina, Sonic, Villager, ROB, Olimar, Pac-Man, Mega Man
Clutch factor / strong reward: ZSS, Ryu, MK, Wario, Lucario / Rosalina, Ness, Fox, Falcon, Luigi, Ike, DK
Noobslayers: Yoshi, Link, Roy, Shulk, Ganondorf
Now I'll have to start explaining myself, these are the characters that are commonly seen as much better characters than what they, in my opinion, really are because of their ability to overwhelm and/or hard punish people who rush in and aren't very experienced either versus those characters or overall. Meaty attacks, especially smashes, that hurt a lot, but if you play patient against them and space and punish properly there's relatively little they can actually get done. I think most people here see these characters for what they are, but this is here for less experienced people (Roy and Ganondorf still get overrated here too though). Even if you're experienced, it's easy to get biased by the sheer power and damage of characters like Ganondorf as a whole or Roy and his smashes and ftilt, but you have to realize that if their options are unsafe (and in the case of Ganondorf also slow) they suffer more and more as the meta advances. Roy basically relies on "bad" spacing, but at the same time he's given the mobility specs to do it, yet his frame data and landing lag isn't nearly what you'd expect for a character that plays rushdown.
Terribad neutral: Zelda, Ganondorf, Jigglypuff
Maybe the worst 3 neutrals in the game, although not going to make that a heavy claim. They're all slow, Zelda in particular has bad hitboxes and unsafe attacks, Ganondorf can't whiff anything and has relative trouble getting grabs, and Jigglypuff has to put herself in the air which is generally a disadvantageous position if you stay there a lot.
Tough disadvantage: Robin, Samus, Roy, Bowser, Charizard, Shulk, Zelda, Ganondorf
Notice that Dedede isn't here, as I feel like he's a special case for a superheavy because of his multiple jumps, lagless nair and disjoints. Landing probably still isn't easy for him, but it's not on the same level as these characters. There are characters like Rosalina, Falcon, ROB and DK who also have severe trouble in disadvantage, but I won't officially list them here because it doesn't keep them from being good due to their strong positive traits. Robin still gets a mention though.
Bad survivability: Mewtwo, Mac, Jigglypuff
These characters may or may not have some things going for them, but then they die. Still, out of these characters I still feel like only Mac could rise beyond lower mid tier if this problem was completely removed. The other two don't have enough strong traits compared to their negative ones (Jigglypuff in particular).
Severe trouble killing: DHD, Lucina, Palutena, Shulk
This is the main reason Lucina is so much lower than Marth. None of her aerials kill before about 155-160%, and most of them take way more than that, fair taking past 200% to kill. Her fsmash and usmash are good, but smashes are not good kill options in general because of how predictable they are unless they're really powerful and (at least fairly) safe at the same time, and that means very few in the game none of which Lucina possesses. Palutena has bair, uair and sort of dash attack, but none of them are particularly strong except uair which is somewhat tough to land due to the position it requires you to be in. Shulk has really slow attacks overall and mostly also relies on smashes to kill, except they're slower than Lucina's. He also has bair and uair, but bair is very slow and uair is quite hard to land. I originally said Marth also struggles to close off stocks, but he's definitely not as bad and I'll also give him the benefit of a doubt and say that a Marth player who plays the character optimally can land tipper aerials and fsmashes so consistently that getting kills is simply not a problem. Duck Hunt gets a special mention because trouble killing is probably the only reason he isn't lower high tier, although I'm not fully aware of the setup DunnoBro discovered.
EDIT: Well, character specific explanations are definitely better and clearer but hey I tried something else. The noobslayers thing though was something I wanted to mention and what also started this idea.