Im going to say the mix of hyper vertical and horizontal mobility thats blindingly fast with effectively no real cooldown, aka its short enough that heāll basically always have it when he needs it sans if he just punches into 5 people with no back up.
The oversized hitbox on punch which gives him pretty free m1ās that do good damage, meaning his damage isnt even skilled, hes still mostly sets himself up for it for free.
His block reducing all front facing damage by 80% on a 7 second cooldown isnt fun either as this character is hard enough to find vulnerable, and power block can basically guarantee himself enough time to get his mobility back up. This wouldnt be such an issue with old Overwatchās CC, but since McCree no longer has flash there isnt a DPS option in the roster that can punish him for reckless aggression, and its basically down solely to who your tank picks on whether or not power block gets infinite value, or is a completely useless ability. You got a Sigma? Good luck as Doom finding power block value. Got a hog? Hog kinda sucks in the meta and isnt super worth going just to counter a doom. Doom facing basically anyone else? Youre so golden youve turned into a god.
And beyond that, youre encouraged to take awful insane risky fights with doom once your ult is up, as its the singular safest ult in the game. Near instantaneous, fastest map traversal tool, and completely immortal now with a heal built in. Did you mess up a play terribly and are finally at risk of dying? Press R and get out of jail free. People say dooms ult is bad, but theyre basically all just bronzies who think āif it doesnt do damage, its worthlessā and cant see the value in having a guaranteed escape from any situation. This character basically has old sombra translocator on steroids.
All of that being said, I dont think this guy is in a terribly good state, these are all just my reasons as to why this character is cringe and will never be fun to fight.
To TLDR this, it can be summed up like this:
More mobility than hammond
has a near immortality level ability in powerblock
Has few counters that counter him too hard, but everyone else can barely play against him
Sets up his own damage too easily, with little counterplay against it
and can escape any dangerous situation when his ult is up, furthering the issue of how unkillable this tank is without specific counters only found in the tank role when not played like an idiot.
I want to reiterate again that I dont think Doom is some overpowered nightmare, because he just isnt. These are all just reasons why heās insufferable to play against and no matter what numbers Blizzard changes, his current kit is too infuriating to fight and would probably find himself banned out a lot (if hero bans get added) solely because no one enjoys a game with a doomfist in it except the doom.