*takes a long deep breath*
Each saga, Ash's stupidity varies, alongside his feel of loss.
Brock's a womanizer.
Misty became a tsundere.
Lt. Surge didn't change much...personality-wise. He was made bigger for some reason.
Erika became a judgmental ***** before she was rectified (somewhat) in Journeys.
Sabrina is a psychic nutjob.
What in god's name happened between Charmander and Charmeleon?
Blaine likes to pretend being a religious hippie.
Jasmine's shyness was completely scrapped (it's pretty much her defining trait and awkwardness around people).
Clair became oh-so changed. She became Lance's respectful superior whereas in truth she's supposed to be Lance's embarrassing hot-headed cousin. Strangely enough, when she came back during DI, her personality again changed that people could mistake her for an older Iris (OLM, make up your mind).
Fuzzy on Flannery. I don't remember if she wasn't a mess like she is in the games.
Fuzzy on Winona too.
Steven's quirkiness for rare stones and his dreamboat-like charms are not even mentioned.
As far as we know, we have no clue if Gardenia has a fear of Ghost-types.
Maylene's ditzy personality was scrapped away because Lucario user.
(Crasher) Wake has a singing career.
Mira's ditziness was scrapped
Roark's dad Byron is an embarrassing dad with a fetish for holes.
Marley's social awkwardness and quiet speech were scrapped.
Cynthia's hidden silly facts were scrapped and replaced by her once-on-a-blue moon ice cream ordering joke (why?)
OLM made Cyrus smile. That alone is way off-the-charts glaring.
...I'm not going to get into excruciating detail how basically almost every Unova character got massive changes here and there, so I'll just shorten the list by saying only Lenora is more or less intact.
Viola became a serious battler, whereas she's always trying to get a good snapshot with her camera and is a bit silly.
Korrina became basically Maylene 2.0, it's not funny. Her roller skate puns were scrapped (which is...incredible, considering the awful puns the dub likes to come up with).
YMMV on Clemont, but he certainly never had inferiority issues in the game.
Bonnie is not obsessed with forcing a marriage onto her brother, and she's supposed to be more competent at gym work than him.
Valerie's insane idea of wanting to become a Pokemon was scrapped. She's just a wide bug-eyed lady who doesn't creep anyone out.
Very minor, but Wulfric doesn't consider himself to be weak.
Lillie's lack of willpower is far less larger than what she started in the games in order to make Ash (and her brother) look more competent.
Kiawe started off as too normal before the episodes went by and he became a caricature of his game self.
Mallow's a superb chef instead of being this quirky chef (that personality went to her Brock-like brother, who's basically a failed Brock expy).
Sophocles's social anxiety was scrapped (notice how often the anime likes scrapping this kind of personality?)
Kukui is nowhere his silly self in the games nor has his verbal tic, except for his Royal Mask persona. (Also, his face looks weird.)
Sweet lord, Olivia, what have they done to you?
Acerola's happy-go-lucky attitude got taken down several notches.
Nanu is far more approachable than in the games.
Due to Hapu not going out into the world to broaden her horizons, she became extremely wary of strangers coming to her home and lacks the wisdom she was known to have.
We all know the embarrassing mom mess Lusamine turned into.
Faba is more or less taken as a silly nuisance rather than this scheming bastard trying to overthrow Lusamine, and he's a Masked Royal fan, which is pretty damning.
Hau behaves nothing like his game self does.
Gladion became discount Alain 2.0.
I'm not sure if Raihan is way too much of a Leon fanatic than in the games.
Hop is pretty much extremely underdeveloped and was reduced to a full-time fanatic for his brother.
Sonia's much more of a mess than in the games so that Goh would utterly outshine her.
We barely saw enough of Marnie to come to a conclusion.
*pants*
Honorable mention: Jealous Journeys Pikachu.
There we go. I think I got them all.