Hitstun.
I think Gold_TSG may offer his input on certain things [ex: this is really boneheaded for a variety of reasons, or help explain design decisions he was a part of].
Part of the problem here is that characters are vastly underexplored, with a few exceptions. I remember when PowerUp was demanding armor on Yoshi's Egg Roll - now I look at BC's Yoshi and wonder what the nerf bat will bring [if anything] in 4.0. Similar things happened with Peach - we all wanted buffs, then you showed us she doesn't suck.
Part of the problem is that certain tricks were lost to the past and need to be rediscovered, part of the problem is that some things were added in for laughs that might not work well, and part of the problem is that some characters are just hard to play or aren't played at all, or else "easy" to play and get nerfed right away, leaving them in an odd spot [Peach is pretty technical, we all know Fox is good but he's very technical, Falco's got some stuff with b-reversed lasers and JC side+B that's mostly unexplored, ROB was explored well enough to nerf him but we don't know if we went too far or not, Kirby is considered broken by Kien but totally fair by me, suggesting we have different benchmarks, Sheik's been nerfed every patch since MAX and she's amazing now, 'Zard was hated by his best player and feared by others (I think unjustly so), Ivysaur received a ton of nerfs, we all have varying opinions on Link, owo and I brought some attention to some silly stuff Falco does, and the list goes on...].
If we had a larger playerbase with people dedicated to each character, we might make progress [part of the PMDT's goal is to let people playtest in tournaments, so to speak], but we don't have too many members, so even though we have nearly every character covered, a lot of it is dual mains (or just multi-mains) and skill gaps can make some characters look broken that maybe aren't (I think the last wave of ROB nerfs was a little much) or vice versa (We had no dedicated Sheiks and though The_Concept joined and had been upset with jump height nerfs previously [and myself for my own reason], we all more or less agree they were well-deserved nowadays...).
It's easy to jump on the spacie bandwagon in Minus or pick someone who bodies them in the combo game, and Falcon, Ganondorf, and Ike are alluring to almost all new players, but so many characters have so much potential that it's rough balancing them out and then we feel some are low-tier when they really might not be [ex: Pikachu, Squirtle, some more...]
